title: Implement a read receipt feature in Firebase group messaging app
body: <p>I'd like to implement a 'Seen' feature in my Firebase group messaging app. Can you kindly advise the best and most efficient approach to take (working code will be appreciated)? For example, the app would show "<strong>Seen by 6</strong>" or "<strong>Seen by 15</strong>" on a group message.</p>
  
  <p>Here's my project: <a href="https://github.com/firebase/friendlychat/tree/master/android" rel="nofollow noreferrer">https://github.com/firebase/friendlychat/tree/master/android</a></p>
  
  <p>Here's the MainActivity: <a href="https://github.com/firebase/friendlychat/blob/master/android/app/src/main/java/com/google/firebase/codelab/friendlychat/MainActivity.java" rel="nofollow noreferrer">https://github.com/firebase/friendlychat/blob/master/android/app/src/main/java/com/google/firebase/codelab/friendlychat/MainActivity.java</a></p>