What do you think of when you hear the word "posse"? I think of movies about the wild west. "Round up a posse and we'll bring that varmint to justice!" Posses have a generally negative connotation. Check out these formal definitions:
Yet whenever she felt like a fish out of water, Wade could turn to her "posse" — nine other girls who, like her, had been recruited from struggling Boston-area school districts and sent on full merit scholarship to the elite women's college.