Posse up -When a large number of people group together; get together to do a common thing; especially for retaliation.
Dey shot at my boy last night. We gotta posse up and git dem!
Posse- group of similar thinking people; from da same hood, school; apartment complex, family or job. Posse up- when uh gang, friends, or groups of people get together to do something as uh whole. For example; fight; especially, when it's in payback for something; Also rob, party, or steal as a group.