When a group of very close-knit friends (male and female) start hooking up with each other in various combinations, often out of convenience. The group could be any group of people who, for whatever reasons, have become close and have their own unit separate from the influences of others outside the group, hence it is almost incestuous for group members to hook up.
**Two bros from a 12 person camping trip are talking**
Guy 1: "Dude, did you hook up with Heather from our camping group?"
Guy 2: "Hell yeah I did! And Sarah and Emily too. Did Emily hook up with Chris?"
Guy 1: "That's what I heard. Dude... this is groupcest, bro."