With iBeacon, you can add other users to a group easily if they’re close by. Make sure your bluetooth is on, and that GroupMe is turned on in Settings > Privacy > Location Services. Then, when you go to add a member in your group as usual, nearby friends appear at the top automatically.
This is a great way to start a new group with people you’re physically near, but may not be in your GroupMe contacts already!
This feature is available on iOS.