Just whatever you do, don't point your finger at them.
Seriously, a lot of it will boil down to who contacts the police first. If you report the assault and your actions first, the police are more likely to focus their attention on the attackers. If they run away and report you first, you're more likely to face problems.
From what I have read, there isn't a clearly defined line for what will get you charged with brandishing or not. It's mostly left to the judgment of the officers at the scene, or the individuals who apply to a magistrate for a warrant after the fact.