Cross between bro + promoter. (as in a club promoter). Its a guy (your bro) who wants you, for inexplicable reasons, to hang with all his other buds, creating an inevitable sausage fest at the party, when girls don't show up (or they were never invited).
It can even apply to football parties, if the bromoter is going apeshit about how cool his bros are.
Man, listen to Dave talking about this guy friends. He's such a bromoter