A Question on large squirmishes and PvP

In a PvP Group v. Group battle, it might get confusing. If it's a bunch of knights, then they all look the same. Perhaps you are a battlemage in the back, waiting to unleash a spell. Who is friend or foe? Can you attack your own group member? If it's more than 2 groups, and you belong to one group, and it's an all-out rumble, how do you keep from attacking your own ally?