Originally, a group of people (especially in the Old West) banded together for purposes of law enforement (and harassing Indians). From posse comitatus, Latin for "power of the county". Posse Comitatus is also the name of various white supremacist organizations that have appeared in the US over the years.
"what you doin?"
"Nothin just chillin with my posse."
"Oh what u doin later?"
"I dunno maybe drive around and pick up guys or drink if we can find someone to buy us liquor."