tried this a while back. called out every freep and creep available a week in advance for a weekend brawl.
TONS of people turned out. too bad it almost always resulted in groups that were WAY larger than 6 just mowing down EVERYONE, and even after they were asked to split their groups they decided that for them it was just more fun to zerg (not ziiiirg :P) everyone around. event lasted only about 1 hour, and i'm pretty sure the only ones that had fun were the large group
stick to a free for all, no holds barred BLOOD FEST>