Supposedly, developers can add the GKMatchmakerViewController function to their games which lets players 'invite' others to play via sending an iMessage in iOS 10 - but that applies to each game individually, and it sounds like might be limited to players saved in a user's contacts, recent players who were seen online, or nearby players on a local network playing the same title? I haven't run across any games yet which prompted me to invite players through GC, though....
