diff --git a/backend/alembic/versions/047_add_is_shared_to_calendars.py b/backend/alembic/versions/047_add_is_shared_to_calendars.py
new file mode 100644
index 0000000..81efe52
--- /dev/null
+++ b/backend/alembic/versions/047_add_is_shared_to_calendars.py
@@ -0,0 +1,23 @@
+"""Add is_shared to calendars
+
+Revision ID: 047
+Revises: 046
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "047"
+down_revision = "046"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "calendars",
+        sa.Column("is_shared", sa.Boolean(), nullable=False, server_default="false"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("calendars", "is_shared")
diff --git a/backend/alembic/versions/048_create_calendar_members.py b/backend/alembic/versions/048_create_calendar_members.py
new file mode 100644
index 0000000..18acfac
--- /dev/null
+++ b/backend/alembic/versions/048_create_calendar_members.py
@@ -0,0 +1,47 @@
+"""Create calendar_members table
+
+Revision ID: 048
+Revises: 047
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "048"
+down_revision = "047"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "calendar_members",
+        sa.Column("id", sa.Integer(), primary_key=True, autoincrement=True),
+        sa.Column("calendar_id", sa.Integer(), sa.ForeignKey("calendars.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("user_id", sa.Integer(), sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("invited_by", sa.Integer(), sa.ForeignKey("users.id", ondelete="SET NULL"), nullable=True),
+        sa.Column("permission", sa.String(20), nullable=False),
+        sa.Column("can_add_others", sa.Boolean(), nullable=False, server_default="false"),
+        sa.Column("local_color", sa.String(20), nullable=True),
+        sa.Column("status", sa.String(20), nullable=False, server_default="pending"),
+        sa.Column("invited_at", sa.DateTime(), nullable=False, server_default=sa.func.now()),
+        sa.Column("accepted_at", sa.DateTime(), nullable=True),
+        sa.UniqueConstraint("calendar_id", "user_id", name="uq_calendar_members_cal_user"),
+        sa.CheckConstraint(
+            "permission IN ('read_only', 'create_modify', 'full_access')",
+            name="ck_calendar_members_permission",
+        ),
+        sa.CheckConstraint(
+            "status IN ('pending', 'accepted', 'rejected')",
+            name="ck_calendar_members_status",
+        ),
+    )
+    op.create_index("ix_calendar_members_user_id", "calendar_members", ["user_id"])
+    op.create_index("ix_calendar_members_calendar_id", "calendar_members", ["calendar_id"])
+    op.create_index("ix_calendar_members_status", "calendar_members", ["status"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_calendar_members_status", table_name="calendar_members")
+    op.drop_index("ix_calendar_members_calendar_id", table_name="calendar_members")
+    op.drop_index("ix_calendar_members_user_id", table_name="calendar_members")
+    op.drop_table("calendar_members")
diff --git a/backend/alembic/versions/049_create_event_locks.py b/backend/alembic/versions/049_create_event_locks.py
new file mode 100644
index 0000000..847ba46
--- /dev/null
+++ b/backend/alembic/versions/049_create_event_locks.py
@@ -0,0 +1,30 @@
+"""Create event_locks table
+
+Revision ID: 049
+Revises: 048
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "049"
+down_revision = "048"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "event_locks",
+        sa.Column("id", sa.Integer(), primary_key=True, autoincrement=True),
+        sa.Column("event_id", sa.Integer(), sa.ForeignKey("calendar_events.id", ondelete="CASCADE"), nullable=False, unique=True),
+        sa.Column("locked_by", sa.Integer(), sa.ForeignKey("users.id", ondelete="CASCADE"), nullable=False),
+        sa.Column("locked_at", sa.DateTime(), nullable=False, server_default=sa.func.now()),
+        sa.Column("expires_at", sa.DateTime(), nullable=True),
+        sa.Column("is_permanent", sa.Boolean(), nullable=False, server_default="false"),
+    )
+    op.create_index("ix_event_locks_expires_at", "event_locks", ["expires_at"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_event_locks_expires_at", table_name="event_locks")
+    op.drop_table("event_locks")
diff --git a/backend/alembic/versions/050_expand_notification_types_calendar.py b/backend/alembic/versions/050_expand_notification_types_calendar.py
new file mode 100644
index 0000000..27df8ac
--- /dev/null
+++ b/backend/alembic/versions/050_expand_notification_types_calendar.py
@@ -0,0 +1,33 @@
+"""Expand notification type CHECK for calendar invite types
+
+Revision ID: 050
+Revises: 049
+"""
+from alembic import op
+
+revision = "050"
+down_revision = "049"
+branch_labels = None
+depends_on = None
+
+_OLD_TYPES = (
+    "connection_request", "connection_accepted", "connection_rejected",
+    "info", "warning", "reminder", "system",
+)
+_NEW_TYPES = _OLD_TYPES + (
+    "calendar_invite", "calendar_invite_accepted", "calendar_invite_rejected",
+)
+
+
+def _check_sql(types: tuple) -> str:
+    return f"type IN ({', '.join(repr(t) for t in types)})"
+
+
+def upgrade() -> None:
+    op.drop_constraint("ck_notifications_type", "notifications", type_="check")
+    op.create_check_constraint("ck_notifications_type", "notifications", _check_sql(_NEW_TYPES))
+
+
+def downgrade() -> None:
+    op.drop_constraint("ck_notifications_type", "notifications", type_="check")
+    op.create_check_constraint("ck_notifications_type", "notifications", _check_sql(_OLD_TYPES))
diff --git a/backend/alembic/versions/051_add_updated_by_to_calendar_events.py b/backend/alembic/versions/051_add_updated_by_to_calendar_events.py
new file mode 100644
index 0000000..7e7a040
--- /dev/null
+++ b/backend/alembic/versions/051_add_updated_by_to_calendar_events.py
@@ -0,0 +1,34 @@
+"""Add updated_by to calendar_events and ensure updated_at index
+
+Revision ID: 051
+Revises: 050
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = "051"
+down_revision = "050"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "calendar_events",
+        sa.Column(
+            "updated_by",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="SET NULL"),
+            nullable=True,
+        ),
+    )
+    op.create_index(
+        "ix_calendar_events_updated_at",
+        "calendar_events",
+        ["updated_at"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_calendar_events_updated_at", table_name="calendar_events")
+    op.drop_column("calendar_events", "updated_by")
diff --git a/backend/app/models/__init__.py b/backend/app/models/__init__.py
index 0b96dc8..81e14b1 100644
--- a/backend/app/models/__init__.py
+++ b/backend/app/models/__init__.py
@@ -18,6 +18,8 @@ from app.models.audit_log import AuditLog
 from app.models.notification import Notification
 from app.models.connection_request import ConnectionRequest
 from app.models.user_connection import UserConnection
+from app.models.calendar_member import CalendarMember
+from app.models.event_lock import EventLock
 
 __all__ = [
     "Settings",
@@ -40,4 +42,6 @@ __all__ = [
     "Notification",
     "ConnectionRequest",
     "UserConnection",
+    "CalendarMember",
+    "EventLock",
 ]
diff --git a/backend/app/models/calendar.py b/backend/app/models/calendar.py
index 43ab782..48c9ab2 100644
--- a/backend/app/models/calendar.py
+++ b/backend/app/models/calendar.py
@@ -1,7 +1,10 @@
 from sqlalchemy import String, Boolean, Integer, ForeignKey, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from datetime import datetime
-from typing import List
+from typing import List, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from app.models.calendar_member import CalendarMember
 from app.database import Base
 
 
@@ -17,7 +20,9 @@ class Calendar(Base):
     is_default: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
     is_system: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
     is_visible: Mapped[bool] = mapped_column(Boolean, default=True, server_default="true")
+    is_shared: Mapped[bool] = mapped_column(Boolean, default=False, server_default="false")
     created_at: Mapped[datetime] = mapped_column(default=func.now())
     updated_at: Mapped[datetime] = mapped_column(default=func.now(), onupdate=func.now())
 
     events: Mapped[List["CalendarEvent"]] = relationship(back_populates="calendar")
+    members: Mapped[List["CalendarMember"]] = relationship(back_populates="calendar", cascade="all, delete-orphan")
diff --git a/backend/app/models/calendar_event.py b/backend/app/models/calendar_event.py
index c85dfc7..2c4f256 100644
--- a/backend/app/models/calendar_event.py
+++ b/backend/app/models/calendar_event.py
@@ -32,6 +32,11 @@ class CalendarEvent(Base):
     # original_start: the originally computed occurrence datetime (children only)
     original_start: Mapped[Optional[datetime]] = mapped_column(nullable=True)
 
+    updated_by: Mapped[Optional[int]] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="SET NULL"), nullable=True
+    )
+
+
     created_at: Mapped[datetime] = mapped_column(default=func.now())
     updated_at: Mapped[datetime] = mapped_column(default=func.now(), onupdate=func.now())
 
diff --git a/backend/app/models/calendar_member.py b/backend/app/models/calendar_member.py
new file mode 100644
index 0000000..56f647f
--- /dev/null
+++ b/backend/app/models/calendar_member.py
@@ -0,0 +1,53 @@
+from sqlalchemy import (
+    Boolean, CheckConstraint, DateTime, Integer, ForeignKey, Index,
+    String, UniqueConstraint, func,
+)
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from datetime import datetime
+from typing import Optional
+from app.database import Base
+
+
+class CalendarMember(Base):
+    __tablename__ = "calendar_members"
+    __table_args__ = (
+        UniqueConstraint("calendar_id", "user_id", name="uq_calendar_members_cal_user"),
+        CheckConstraint(
+            "permission IN ('read_only', 'create_modify', 'full_access')",
+            name="ck_calendar_members_permission",
+        ),
+        CheckConstraint(
+            "status IN ('pending', 'accepted', 'rejected')",
+            name="ck_calendar_members_status",
+        ),
+        Index("ix_calendar_members_user_id", "user_id"),
+        Index("ix_calendar_members_calendar_id", "calendar_id"),
+        Index("ix_calendar_members_status", "status"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    calendar_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("calendars.id", ondelete="CASCADE"), nullable=False
+    )
+    user_id: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    invited_by: Mapped[Optional[int]] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="SET NULL"), nullable=True
+    )
+    permission: Mapped[str] = mapped_column(String(20), nullable=False)
+    can_add_others: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false"
+    )
+    local_color: Mapped[Optional[str]] = mapped_column(String(20), nullable=True)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="pending")
+    invited_at: Mapped[datetime] = mapped_column(
+        DateTime, default=func.now(), server_default=func.now()
+    )
+    accepted_at: Mapped[Optional[datetime]] = mapped_column(DateTime, nullable=True)
+
+    calendar: Mapped["Calendar"] = relationship(back_populates="members", lazy="selectin")
+    user: Mapped["User"] = relationship(foreign_keys=[user_id], lazy="selectin")
+    inviter: Mapped[Optional["User"]] = relationship(
+        foreign_keys=[invited_by], lazy="selectin"
+    )
diff --git a/backend/app/models/event_lock.py b/backend/app/models/event_lock.py
new file mode 100644
index 0000000..79bb05b
--- /dev/null
+++ b/backend/app/models/event_lock.py
@@ -0,0 +1,31 @@
+from sqlalchemy import Boolean, DateTime, Integer, ForeignKey, Index, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from datetime import datetime
+from typing import Optional
+from app.database import Base
+
+
+class EventLock(Base):
+    __tablename__ = "event_locks"
+    __table_args__ = (Index("ix_event_locks_expires_at", "expires_at"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    event_id: Mapped[int] = mapped_column(
+        Integer,
+        ForeignKey("calendar_events.id", ondelete="CASCADE"),
+        nullable=False,
+        unique=True,
+    )
+    locked_by: Mapped[int] = mapped_column(
+        Integer, ForeignKey("users.id", ondelete="CASCADE"), nullable=False
+    )
+    locked_at: Mapped[datetime] = mapped_column(
+        DateTime, default=func.now(), server_default=func.now()
+    )
+    expires_at: Mapped[Optional[datetime]] = mapped_column(DateTime, nullable=True)
+    is_permanent: Mapped[bool] = mapped_column(
+        Boolean, default=False, server_default="false"
+    )
+
+    event: Mapped["CalendarEvent"] = relationship(lazy="selectin")
+    holder: Mapped["User"] = relationship(foreign_keys=[locked_by], lazy="selectin")
diff --git a/backend/app/models/notification.py b/backend/app/models/notification.py
index 58d71c3..2f4cfc6 100644
--- a/backend/app/models/notification.py
+++ b/backend/app/models/notification.py
@@ -5,10 +5,9 @@ from datetime import datetime
 from typing import Optional
 from app.database import Base
 
-# Active: connection_request, connection_accepted
-# Reserved: connection_rejected, info, warning, reminder, system
 _NOTIFICATION_TYPES = (
     "connection_request", "connection_accepted", "connection_rejected",
+    "calendar_invite", "calendar_invite_accepted", "calendar_invite_rejected",
     "info", "warning", "reminder", "system",
 )
 
diff --git a/backend/app/schemas/calendar.py b/backend/app/schemas/calendar.py
index e9e2753..15da21f 100644
--- a/backend/app/schemas/calendar.py
+++ b/backend/app/schemas/calendar.py
@@ -8,6 +8,7 @@ class CalendarCreate(BaseModel):
 
     name: str = Field(min_length=1, max_length=100)
     color: str = Field("#3b82f6", max_length=20)
+    is_shared: bool = False
 
 
 class CalendarUpdate(BaseModel):
@@ -16,6 +17,7 @@ class CalendarUpdate(BaseModel):
     name: Optional[str] = Field(None, min_length=1, max_length=100)
     color: Optional[str] = Field(None, max_length=20)
     is_visible: Optional[bool] = None
+    is_shared: Optional[bool] = None
 
 
 class CalendarResponse(BaseModel):
@@ -27,5 +29,11 @@ class CalendarResponse(BaseModel):
     is_visible: bool
     created_at: datetime
     updated_at: datetime
+    is_shared: bool = False
+    owner_umbral_name: Optional[str] = None
+    my_permission: Optional[str] = None
+    my_can_add_others: bool = False
+    my_local_color: Optional[str] = None
+    member_count: int = 0
 
     model_config = ConfigDict(from_attributes=True)
diff --git a/backend/app/schemas/shared_calendar.py b/backend/app/schemas/shared_calendar.py
new file mode 100644
index 0000000..5b5420f
--- /dev/null
+++ b/backend/app/schemas/shared_calendar.py
@@ -0,0 +1,79 @@
+import re
+from pydantic import BaseModel, ConfigDict, Field, field_validator
+from typing import Optional, Literal
+from datetime import datetime
+
+
+class InviteMemberRequest(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+    connection_id: int = Field(ge=1, le=2147483647)
+    permission: Literal["read_only", "create_modify", "full_access"]
+    can_add_others: bool = False
+
+
+class RespondInviteRequest(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+    action: Literal["accept", "reject"]
+
+
+class UpdateMemberRequest(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+    permission: Optional[Literal["read_only", "create_modify", "full_access"]] = None
+    can_add_others: Optional[bool] = None
+
+
+class UpdateLocalColorRequest(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+    local_color: Optional[str] = Field(None, max_length=20)
+
+    @field_validator("local_color")
+    @classmethod
+    def validate_color(cls, v: Optional[str]) -> Optional[str]:
+        if v is not None and not re.match(r"^#[0-9a-fA-F]{6}$", v):
+            raise ValueError("Color must be a hex color code (#RRGGBB)")
+        return v
+
+
+class ConvertToSharedRequest(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
+
+class CalendarMemberResponse(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    calendar_id: int
+    user_id: int
+    umbral_name: str
+    preferred_name: Optional[str] = None
+    permission: str
+    can_add_others: bool
+    local_color: Optional[str] = None
+    status: str
+    invited_at: datetime
+    accepted_at: Optional[datetime] = None
+
+
+class CalendarInviteResponse(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    calendar_id: int
+    calendar_name: str
+    calendar_color: str
+    owner_umbral_name: str
+    inviter_umbral_name: str
+    permission: str
+    invited_at: datetime
+
+
+class LockStatusResponse(BaseModel):
+    locked: bool
+    locked_by_name: Optional[str] = None
+    expires_at: Optional[datetime] = None
+    is_permanent: bool = False
+
+
+class SyncResponse(BaseModel):
+    events: list[dict]
+    member_changes: list[dict]
+    server_time: datetime
+    truncated: bool = False