Ok. Since everyone is talking stag-dos. Here is a fun one that could happen (could happen in fanfiction). 

There is a joint stag do (like Robert and Aaron have more friends) and they go with the girl squad and any other random characters. It’s the same club/bar they were at during the reunion episode. While Aaron is off getting more drinks Mike shows up. Everyone just smiles at him while looking for Aaron nervously. 

Robert and Mike talk and Mike asks about the ring only to have Aaron show up and say something snarky while grabbing Roberts elbow to pull him away. Whispers something dirty in Roberts’ ear. The girl squad doesn’t see them for a while.