add trigger for updated user

functions/src/on-update-user.ts

@@ -0,0 +1,18 @@
+import * as functions from 'firebase-functions'
+import * as admin from 'firebase-admin'
+import { User } from '../../common/user'
+const firestore = admin.firestore()
+
+export const onUpdateGroup = functions.firestore
+  .document('users/{userId}')
+  .onUpdate(async (change) => {
+    const prevUser = change.before.data() as User
+    const user = change.after.data() as User
+
+    // if they're updating their referredId, send the
+
+    await firestore
+      .collection('groups')
+      .doc(group.id)
+      .update({ mostRecentActivityTime: Date.now() })
+  })