The act of participating in an extremely fun activity consisting of 3 or more people. Usually, a couple will be involved, in which the GIRLFRIEND to THE COUPLE does not agree with GROUP PARTY THING, but still decides to go along with her fellow mates desires.
Dude: Hey...would you like to go on a cruise????
Dudette: YES...I'VE ALWALYS WANTED TO GO ON A CRUISE!!!!!!!!!
Dude: Yeah...well let's make it a GROUP PARTY THING!
Dudette: Huh...Group...PARTY THING?!?!?
Dudette: Ok...sounds cool, I'm down for anything...