diff --git a/src/db.ts b/src/db.ts
index 879fd3b..4081380 100644
--- a/src/db.ts
+++ b/src/db.ts
@@ -60,6 +60,32 @@ export function initializeDatabase(instance: Database) {
     );
   `);
 
+  // Create group_members table
+  instance.exec(`
+    CREATE TABLE IF NOT EXISTS group_members (
+      group_id TEXT NOT NULL,
+      user_id TEXT NOT NULL,
+      is_admin BOOLEAN NOT NULL DEFAULT 0,
+      is_active BOOLEAN NOT NULL DEFAULT 1,
+      first_seen_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%d %H:%M:%f', 'now')),
+      last_seen_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%d %H:%M:%f', 'now')),
+      last_role_change_at TEXT NULL,
+      PRIMARY KEY (group_id, user_id),
+      FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE,
+      FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
+    );
+  `);
+
+  // Indexes for membership lookups
+  instance.exec(`
+    CREATE INDEX IF NOT EXISTS idx_group_members_group_active
+    ON group_members (group_id, is_active);
+  `);
+  instance.exec(`
+    CREATE INDEX IF NOT EXISTS idx_group_members_user_active
+    ON group_members (user_id, is_active);
+  `);
+
   // Create tasks table
   instance.exec(`
     CREATE TABLE IF NOT EXISTS tasks (
diff --git a/src/db/migrations/index.ts b/src/db/migrations/index.ts
index 95e74b1..61b2ede 100644
--- a/src/db/migrations/index.ts
+++ b/src/db/migrations/index.ts
@@ -147,5 +147,35 @@ export const migrations: Migration[] = [
         ON user_preferences (reminder_freq, reminder_time);
       `);
     }
+  },
+  {
+    version: 5,
+    name: 'group-membership',
+    checksum: 'v5-group-membership-2025-09-07',
+    up: (db: Database) => {
+      db.exec(`PRAGMA foreign_keys = ON;`);
+      db.exec(`
+        CREATE TABLE IF NOT EXISTS group_members (
+          group_id TEXT NOT NULL,
+          user_id TEXT NOT NULL,
+          is_admin BOOLEAN NOT NULL DEFAULT 0,
+          is_active BOOLEAN NOT NULL DEFAULT 1,
+          first_seen_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%d %H:%M:%f', 'now')),
+          last_seen_at TEXT NOT NULL DEFAULT (strftime('%Y-%m-%d %H:%M:%f', 'now')),
+          last_role_change_at TEXT NULL,
+          PRIMARY KEY (group_id, user_id),
+          FOREIGN KEY (group_id) REFERENCES groups(id) ON DELETE CASCADE,
+          FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
+        );
+      `);
+      db.exec(`
+        CREATE INDEX IF NOT EXISTS idx_group_members_group_active
+        ON group_members (group_id, is_active);
+      `);
+      db.exec(`
+        CREATE INDEX IF NOT EXISTS idx_group_members_user_active
+        ON group_members (user_id, is_active);
+      `);
+    }
   }
 ];
diff --git a/src/services/group-sync.ts b/src/services/group-sync.ts
index 5e550b1..139af23 100644
--- a/src/services/group-sync.ts
+++ b/src/services/group-sync.ts
@@ -1,5 +1,6 @@
 import type { Database } from 'bun:sqlite';
-import { db } from '../db';
+import { db, ensureUserExists } from '../db';
+import { normalizeWhatsAppId } from '../utils/whatsapp';
 // Environment variables will be mocked in tests
 const env = process.env;
 
@@ -336,4 +337,178 @@ export class GroupSyncService {
 	static isGroupActive(groupId: string): boolean {
 		return this.activeGroupsCache.has(groupId);
 	}
+
+	// Fetch members for a single group from Evolution API. Uses a robust parser to accept multiple payload shapes.
+	private static async fetchGroupMembersFromAPI(groupId: string): Promise<Array<{ userId: string; isAdmin: boolean }>> {
+		const url = `${env.EVOLUTION_API_URL}/group/fetchAllGroups/${env.EVOLUTION_API_INSTANCE}?getParticipants=true`;
+		console.log('ℹ️ Fetching members via fetchAllGroups (participants=true):', { groupId });
+
+		const response = await fetch(url, {
+			method: 'GET',
+			headers: { apikey: env.EVOLUTION_API_KEY },
+			httpVersion: '2',
+			timeout: 320000
+		});
+		if (!response.ok) {
+			const body = await response.text().catch(() => '');
+			throw new Error(`Failed to fetch groups with participants: ${response.status} ${response.statusText} - ${body.slice(0,200)}`);
+		}
+		const raw = await response.text();
+		let parsed: any;
+		try {
+			parsed = JSON.parse(raw);
+		} catch (e) {
+			console.error('❌ Failed to parse members response JSON:', String(e));
+			throw e;
+		}
+
+		let groups: any[] = [];
+		if (Array.isArray(parsed)) {
+			groups = parsed;
+		} else if (parsed && Array.isArray(parsed.response)) {
+			groups = parsed.response;
+		} else {
+			throw new Error('Invalid response format for groups with participants');
+		}
+
+		const g = groups.find((g: any) => g?.id === groupId);
+		if (!g) {
+			console.warn(`⚠️ Group ${groupId} not present in fetchAllGroups(getParticipants=true) response`);
+			return [];
+		}
+		const participants = Array.isArray(g.participants) ? g.participants : [];
+
+		const result: Array<{ userId: string; isAdmin: boolean }> = [];
+		for (const p of participants) {
+			let jid: string | null = null;
+			let isAdmin = false;
+
+			if (typeof p === 'string') {
+				jid = p;
+			} else if (p && typeof p === 'object') {
+				jid = p.id || p.jid || p.user || p?.user?.id || null;
+				if (typeof p.isAdmin === 'boolean') {
+					isAdmin = p.isAdmin;
+				} else if (typeof p.admin === 'string') {
+					// common shapes: 'admin', 'superadmin' or null
+					isAdmin = p.admin === 'admin' || p.admin === 'superadmin';
+				} else if (typeof p.role === 'string') {
+					isAdmin = p.role.toLowerCase().includes('admin');
+				}
+			}
+
+			const norm = normalizeWhatsAppId(jid);
+			if (!norm) continue;
+			result.push({ userId: norm, isAdmin });
+		}
+		return result;
+	}
+
+	/**
+	 * Reconciles current DB membership state for a group with a fresh snapshot.
+	 * Idempotente y atómico por grupo.
+	 */
+	static reconcileGroupMembers(groupId: string, snapshot: Array<{ userId: string; isAdmin: boolean }>, nowIso?: string): { added: number; updated: number; deactivated: number } {
+		if (!groupId || !Array.isArray(snapshot)) {
+			throw new Error('Invalid arguments for reconcileGroupMembers');
+		}
+		const now = nowIso || new Date().toISOString().replace('T', ' ').replace('Z', '');
+		let added = 0, updated = 0, deactivated = 0;
+
+		// Build quick lookup from snapshot
+		const incoming = new Map<string, { isAdmin: boolean }>();
+		for (const m of snapshot) {
+			if (!m?.userId) continue;
+			incoming.set(m.userId, { isAdmin: !!m.isAdmin });
+		}
+
+		this.dbInstance.transaction(() => {
+			// Load existing membership for group
+			const existingRows = this.dbInstance.prepare(`
+				SELECT user_id, is_admin, is_active 
+				FROM group_members 
+				WHERE group_id = ?
+			`).all(groupId) as Array<{ user_id: string; is_admin: number; is_active: number }>;
+
+			const existing = new Map(existingRows.map(r => [r.user_id, { isAdmin: !!r.is_admin, isActive: !!r.is_active }]));
+
+			// Upsert present members
+			for (const [userId, { isAdmin }] of incoming.entries()) {
+				// Ensure user exists (FK)
+				ensureUserExists(userId, this.dbInstance);
+
+				const row = existing.get(userId);
+				if (!row) {
+					// insert
+					this.dbInstance.prepare(`
+						INSERT INTO group_members (group_id, user_id, is_admin, is_active, first_seen_at, last_seen_at)
+						VALUES (?, ?, ?, 1, ?, ?)
+					`).run(groupId, userId, isAdmin ? 1 : 0, now, now);
+					added++;
+				} else {
+					// update if needed
+					let roleChanged = row.isAdmin !== isAdmin;
+					if (!row.isActive || roleChanged) {
+						this.dbInstance.prepare(`
+							UPDATE group_members
+							SET is_active = 1,
+							    is_admin = ?,
+							    last_seen_at = ?,
+							    last_role_change_at = CASE WHEN ? THEN ? ELSE last_role_change_at END
+							WHERE group_id = ? AND user_id = ?
+						`).run(isAdmin ? 1 : 0, now, roleChanged ? 1 : 0, roleChanged ? now : null, groupId, userId);
+						updated++;
+					} else {
+						// still update last_seen_at to reflect presence
+						this.dbInstance.prepare(`
+							UPDATE group_members
+							SET last_seen_at = ?
+							WHERE group_id = ? AND user_id = ?
+						`).run(now, groupId, userId);
+					}
+				}
+			}
+
+			// Deactivate absent members
+			for (const [userId, state] of existing.entries()) {
+				if (!incoming.has(userId) && state.isActive) {
+					this.dbInstance.prepare(`
+						UPDATE group_members
+						SET is_active = 0,
+						    last_seen_at = ?
+						WHERE group_id = ? AND user_id = ?
+					`).run(now, groupId, userId);
+					deactivated++;
+				}
+			}
+		})();
+
+		return { added, updated, deactivated };
+	}
+
+	/**
+	 * Sync members for all active groups by calling Evolution API and reconciling.
+	 * Devuelve contadores agregados.
+	 */
+	static async syncMembersForActiveGroups(): Promise<{ groups: number; added: number; updated: number; deactivated: number }> {
+		// ensure cache is populated
+		if (this.activeGroupsCache.size === 0) {
+			this.cacheActiveGroups();
+		}
+		let groups = 0, added = 0, updated = 0, deactivated = 0;
+		for (const [groupId] of this.activeGroupsCache.entries()) {
+			try {
+				const snapshot = await this.fetchGroupMembersFromAPI(groupId);
+				const res = this.reconcileGroupMembers(groupId, snapshot);
+				groups++;
+				added += res.added;
+				updated += res.updated;
+				deactivated += res.deactivated;
+			} catch (e) {
+				console.error(`❌ Failed to sync members for group ${groupId}:`, e instanceof Error ? e.message : String(e));
+			}
+		}
+		console.log('ℹ️ Members sync summary:', { groups, added, updated, deactivated });
+		return { groups, added, updated, deactivated };
+	}
 }