Replace "tick marks" on the right lower corner of a message with profile pictures of anyone who has seen the message (alike in
At the moment you can't know easily who has seen/read the message because when someone sees it, two "tick marks" appear to the lower right corner of the message. Of course you can check who's online in your group right that moment, but you have to use a small amount of time for this.
Also you can't really say has everyone seen the message while you have been gone.
Of course this might be stupid in big groups where there might be tens of pictures in the lower right corner, so another option would be to replace "tick marks" with a link to different "who has seen the message"-list when group has more than 5 or 10 people.