A group of girls going to the bathroom together because it's not cool to go by yourself.
Girl 1: I gotta go to the bathroom
Girl 2: Me too, any one else?
Girls 3: Yeah, lets make it a posse piss
When a group of girls all go to the bathroom together
(At a reastaurant)
Girl 1: I'll be back in a min. I have to use the bathroom
Girl 2: Oh wait for me, I do too!
Girl 3: Me too, Let's have a posse piss