diff --git a/src/lib/server/api-utils.ts b/src/lib/server/api-utils.ts
index 3b05fd9..73bde53 100644
--- a/src/lib/server/api-utils.ts
+++ b/src/lib/server/api-utils.ts
@@ -1,4 +1,7 @@
 import { error } from '@sveltejs/kit';
+import { db } from './db/index.js';
+import { notes, noteCollaborators } from './db/schema.js';
+import { eq, and } from 'drizzle-orm';
 
 type EventWithLocals = {
 	locals: App.Locals;
@@ -14,3 +17,36 @@ export function requireAdmin(event: EventWithLocals) {
 	if (!user || user.role !== 'admin') throw error(403, 'Forbidden');
 	return user;
 }
+
+/**
+ * Check if a user can access a note (as owner or collaborator).
+ */
+export function canAccessNote(noteId: string, userId: number): { canAccess: boolean; isOwner: boolean } {
+	const note = db.select({ userId: notes.userId }).from(notes).where(eq(notes.id, noteId)).get();
+	if (!note) return { canAccess: false, isOwner: false };
+	if (note.userId === userId) return { canAccess: true, isOwner: true };
+
+	const collab = db
+		.select({ userId: noteCollaborators.userId })
+		.from(noteCollaborators)
+		.where(and(eq(noteCollaborators.noteId, noteId), eq(noteCollaborators.userId, userId)))
+		.get();
+	return { canAccess: !!collab, isOwner: false };
+}
+
+/**
+ * Require that a user can access a note. Throws 404 if no access.
+ */
+export function requireNoteAccess(noteId: string, userId: number): { isOwner: boolean } {
+	const { canAccess, isOwner } = canAccessNote(noteId, userId);
+	if (!canAccess) throw error(404, 'Not found');
+	return { isOwner };
+}
+
+/**
+ * Require that a user is the owner of a note. Throws 404 if not owner.
+ */
+export function requireNoteOwnership(noteId: string, userId: number): void {
+	const note = db.select({ userId: notes.userId }).from(notes).where(eq(notes.id, noteId)).get();
+	if (!note || note.userId !== userId) throw error(404, 'Not found');
+}
diff --git a/src/lib/server/collaborators.ts b/src/lib/server/collaborators.ts
new file mode 100644
index 0000000..c9d906d
--- /dev/null
+++ b/src/lib/server/collaborators.ts
@@ -0,0 +1,75 @@
+import { db } from './db/index.js';
+import { noteCollaborators, noteUserState, users } from './db/schema.js';
+import { eq, and, inArray } from 'drizzle-orm';
+import type { Collaborator } from '$lib/types/index.js';
+
+/**
+ * Batch fetch collaborators for multiple notes.
+ * Returns a Map of noteId → Collaborator[].
+ */
+export function fetchCollaboratorsForNotes(noteIds: string[]): Map<string, Collaborator[]> {
+	if (noteIds.length === 0) return new Map();
+
+	const rows = db
+		.select({
+			noteId: noteCollaborators.noteId,
+			userId: noteCollaborators.userId,
+			displayName: users.displayName,
+			email: users.email,
+			addedAt: noteCollaborators.addedAt
+		})
+		.from(noteCollaborators)
+		.innerJoin(users, eq(noteCollaborators.userId, users.id))
+		.where(inArray(noteCollaborators.noteId, noteIds))
+		.all();
+
+	const map = new Map<string, Collaborator[]>();
+	for (const row of rows) {
+		const collab: Collaborator = {
+			userId: row.userId,
+			displayName: row.displayName,
+			email: row.email,
+			addedAt: row.addedAt
+		};
+		const existing = map.get(row.noteId);
+		if (existing) {
+			existing.push(collab);
+		} else {
+			map.set(row.noteId, [collab]);
+		}
+	}
+	return map;
+}
+
+/**
+ * Add a collaborator to a note.
+ */
+export function addCollaborator(noteId: string, userId: number, addedBy: number): void {
+	db.insert(noteCollaborators)
+		.values({ noteId, userId, addedBy, addedAt: new Date() })
+		.run();
+}
+
+/**
+ * Remove a collaborator from a note. Also cleans up their per-user state.
+ */
+export function removeCollaborator(noteId: string, userId: number): void {
+	db.delete(noteCollaborators)
+		.where(and(eq(noteCollaborators.noteId, noteId), eq(noteCollaborators.userId, userId)))
+		.run();
+	db.delete(noteUserState)
+		.where(and(eq(noteUserState.noteId, noteId), eq(noteUserState.userId, userId)))
+		.run();
+}
+
+/**
+ * Get all collaborator user IDs for a note.
+ */
+export function getCollaboratorIds(noteId: string): number[] {
+	return db
+		.select({ userId: noteCollaborators.userId })
+		.from(noteCollaborators)
+		.where(eq(noteCollaborators.noteId, noteId))
+		.all()
+		.map((r) => r.userId);
+}
diff --git a/src/lib/server/db/index.ts b/src/lib/server/db/index.ts
index 3e488af..70f07bc 100644
--- a/src/lib/server/db/index.ts
+++ b/src/lib/server/db/index.ts
@@ -93,6 +93,21 @@ sqlite.exec(`
 		client_id TEXT NOT NULL
 	);
 
+	CREATE TABLE IF NOT EXISTS note_collaborators (
+		note_id TEXT NOT NULL REFERENCES notes(id) ON DELETE CASCADE,
+		user_id INTEGER NOT NULL REFERENCES users(id),
+		added_by INTEGER NOT NULL REFERENCES users(id),
+		added_at INTEGER NOT NULL
+	);
+
+	CREATE TABLE IF NOT EXISTS note_user_state (
+		note_id TEXT NOT NULL REFERENCES notes(id) ON DELETE CASCADE,
+		user_id INTEGER NOT NULL REFERENCES users(id),
+		pinned INTEGER NOT NULL DEFAULT 0,
+		archived INTEGER NOT NULL DEFAULT 0,
+		sort_order INTEGER NOT NULL DEFAULT 0
+	);
+
 	CREATE TABLE IF NOT EXISTS login_attempts (
 		id INTEGER PRIMARY KEY AUTOINCREMENT,
 		ip TEXT NOT NULL,
@@ -168,6 +183,10 @@ sqlite.exec(`
 	CREATE INDEX IF NOT EXISTS sync_log_timestamp_idx ON sync_log(timestamp);
 	CREATE INDEX IF NOT EXISTS login_attempts_ip_timestamp_idx ON login_attempts(ip, timestamp);
 
+	CREATE UNIQUE INDEX IF NOT EXISTS note_collaborators_unique ON note_collaborators(note_id, user_id);
+	CREATE INDEX IF NOT EXISTS note_collaborators_user_id_idx ON note_collaborators(user_id);
+	CREATE UNIQUE INDEX IF NOT EXISTS note_user_state_unique ON note_user_state(note_id, user_id);
+
 	CREATE TABLE IF NOT EXISTS api_keys (
 		id TEXT PRIMARY KEY,
 		user_id INTEGER NOT NULL REFERENCES users(id),
diff --git a/src/lib/server/db/schema.ts b/src/lib/server/db/schema.ts
index 49153f9..4dd8c1f 100644
--- a/src/lib/server/db/schema.ts
+++ b/src/lib/server/db/schema.ts
@@ -128,6 +128,42 @@ export const syncLog = sqliteTable(
 	(table) => [index('sync_log_timestamp_idx').on(table.timestamp)]
 );
 
+export const noteCollaborators = sqliteTable(
+	'note_collaborators',
+	{
+		noteId: text('note_id')
+			.references(() => notes.id, { onDelete: 'cascade' })
+			.notNull(),
+		userId: integer('user_id')
+			.references(() => users.id)
+			.notNull(),
+		addedBy: integer('added_by')
+			.references(() => users.id)
+			.notNull(),
+		addedAt: integer('added_at', { mode: 'timestamp' }).notNull()
+	},
+	(table) => [
+		uniqueIndex('note_collaborators_unique').on(table.noteId, table.userId),
+		index('note_collaborators_user_id_idx').on(table.userId)
+	]
+);
+
+export const noteUserState = sqliteTable(
+	'note_user_state',
+	{
+		noteId: text('note_id')
+			.references(() => notes.id, { onDelete: 'cascade' })
+			.notNull(),
+		userId: integer('user_id')
+			.references(() => users.id)
+			.notNull(),
+		pinned: integer('pinned', { mode: 'boolean' }).default(false).notNull(),
+		archived: integer('archived', { mode: 'boolean' }).default(false).notNull(),
+		sortOrder: integer('sort_order').default(0).notNull()
+	},
+	(table) => [uniqueIndex('note_user_state_unique').on(table.noteId, table.userId)]
+);
+
 export const loginAttempts = sqliteTable(
 	'login_attempts',
 	{
diff --git a/src/lib/server/db/test-helpers.ts b/src/lib/server/db/test-helpers.ts
index f045213..6714faa 100644
--- a/src/lib/server/db/test-helpers.ts
+++ b/src/lib/server/db/test-helpers.ts
@@ -68,6 +68,24 @@ export function createTestDb(options?: { seedUser?: boolean }) {
 		CREATE INDEX note_tags_note_id_idx ON note_tags(note_id);
 		CREATE INDEX note_tags_tag_id_idx ON note_tags(tag_id);
 
+		CREATE TABLE note_collaborators (
+			note_id TEXT NOT NULL REFERENCES notes(id) ON DELETE CASCADE,
+			user_id INTEGER NOT NULL REFERENCES users(id),
+			added_by INTEGER NOT NULL REFERENCES users(id),
+			added_at INTEGER NOT NULL
+		);
+		CREATE UNIQUE INDEX note_collaborators_unique ON note_collaborators(note_id, user_id);
+		CREATE INDEX note_collaborators_user_id_idx ON note_collaborators(user_id);
+
+		CREATE TABLE note_user_state (
+			note_id TEXT NOT NULL REFERENCES notes(id) ON DELETE CASCADE,
+			user_id INTEGER NOT NULL REFERENCES users(id),
+			pinned INTEGER NOT NULL DEFAULT 0,
+			archived INTEGER NOT NULL DEFAULT 0,
+			sort_order INTEGER NOT NULL DEFAULT 0
+		);
+		CREATE UNIQUE INDEX note_user_state_unique ON note_user_state(note_id, user_id);
+
 		CREATE TABLE attachments (
 			id TEXT PRIMARY KEY,
 			user_id INTEGER NOT NULL DEFAULT 0 REFERENCES users(id),
diff --git a/src/lib/types/index.ts b/src/lib/types/index.ts
index 8fe9034..47af735 100644
--- a/src/lib/types/index.ts
+++ b/src/lib/types/index.ts
@@ -14,6 +14,16 @@ export interface Note {
 	version: number;
 	tags?: string[];
 	attachments?: Attachment[];
+	collaborators?: Collaborator[];
+	isOwner?: boolean;
+	isShared?: boolean;
+}
+
+export interface Collaborator {
+	userId: number;
+	displayName: string;
+	email: string;
+	addedAt: Date;
 }
 
 export interface NoteCreate {
diff --git a/src/routes/api/notes/[id]/collaborators/+server.ts b/src/routes/api/notes/[id]/collaborators/+server.ts
new file mode 100644
index 0000000..589a4e6
--- /dev/null
+++ b/src/routes/api/notes/[id]/collaborators/+server.ts
@@ -0,0 +1,72 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types.js';
+import { getUserId, requireNoteAccess, requireNoteOwnership } from '$lib/server/api-utils.js';
+import { addCollaborator, removeCollaborator, fetchCollaboratorsForNotes } from '$lib/server/collaborators.js';
+import { db } from '$lib/server/db/index.js';
+import { users, notes, noteCollaborators } from '$lib/server/db/schema.js';
+import { eq, and } from 'drizzle-orm';
+
+export const GET: RequestHandler = async ({ params, ...event }) => {
+	const userId = getUserId(event);
+	requireNoteAccess(params.id, userId);
+
+	const collaborators = fetchCollaboratorsForNotes([params.id]);
+	return json(collaborators.get(params.id) ?? []);
+};
+
+export const POST: RequestHandler = async ({ params, request, ...event }) => {
+	const userId = getUserId(event);
+	requireNoteOwnership(params.id, userId);
+
+	const body = await request.json();
+	const targetUserId = body.userId;
+	if (!targetUserId || typeof targetUserId !== 'number') {
+		throw error(400, 'userId is required');
+	}
+
+	// Cannot add yourself
+	const note = db.select({ userId: notes.userId }).from(notes).where(eq(notes.id, params.id)).get();
+	if (note && note.userId === targetUserId) {
+		throw error(400, 'Cannot add the owner as a collaborator');
+	}
+
+	// Target user must exist
+	const targetUser = db.select({ id: users.id }).from(users).where(eq(users.id, targetUserId)).get();
+	if (!targetUser) {
+		throw error(404, 'User not found');
+	}
+
+	// Check not already a collaborator
+	const existing = db
+		.select({ userId: noteCollaborators.userId })
+		.from(noteCollaborators)
+		.where(and(eq(noteCollaborators.noteId, params.id), eq(noteCollaborators.userId, targetUserId)))
+		.get();
+	if (existing) {
+		throw error(409, 'User is already a collaborator');
+	}
+
+	addCollaborator(params.id, targetUserId, userId);
+
+	const collaborators = fetchCollaboratorsForNotes([params.id]);
+	return json(collaborators.get(params.id) ?? [], { status: 201 });
+};
+
+export const DELETE: RequestHandler = async ({ params, url, ...event }) => {
+	const userId = getUserId(event);
+	const targetUserId = Number(url.searchParams.get('userId'));
+	if (!targetUserId) {
+		throw error(400, 'userId query parameter is required');
+	}
+
+	const { isOwner } = requireNoteAccess(params.id, userId);
+
+	// Owner can remove anyone. Collaborator can only remove self.
+	if (!isOwner && targetUserId !== userId) {
+		throw error(403, 'Only the owner can remove other collaborators');
+	}
+
+	removeCollaborator(params.id, targetUserId);
+
+	return json({ success: true });
+};
diff --git a/src/routes/api/users/search/+server.ts b/src/routes/api/users/search/+server.ts
new file mode 100644
index 0000000..5425aa1
--- /dev/null
+++ b/src/routes/api/users/search/+server.ts
@@ -0,0 +1,35 @@
+import { json } from '@sveltejs/kit';
+import type { RequestHandler } from './$types.js';
+import { getUserId } from '$lib/server/api-utils.js';
+import { db } from '$lib/server/db/index.js';
+import { users } from '$lib/server/db/schema.js';
+import { ne } from 'drizzle-orm';
+
+export const GET: RequestHandler = async ({ url, ...event }) => {
+	const userId = getUserId(event);
+	const query = url.searchParams.get('q')?.trim();
+	if (!query || query.length < 1) {
+		return json([]);
+	}
+
+	const pattern = `%${query}%`;
+	const results = db
+		.select({
+			id: users.id,
+			displayName: users.displayName,
+			email: users.email
+		})
+		.from(users)
+		.where(
+			ne(users.id, userId)
+		)
+		.all()
+		.filter(
+			(u) =>
+				u.displayName.toLowerCase().includes(query.toLowerCase()) ||
+				u.email.toLowerCase().includes(query.toLowerCase())
+		)
+		.slice(0, 10);
+
+	return json(results);
+};