Guild names or anything else probably won't work since players can find work arounds to exploit it. So I figure the simplest solution the monster will have a Radius of like 50 feet. It will count all players in that area. If it counts 2 players then it's damage, defense/resist will be the same. But if it counts just 1 player then it will deal reduced damage and have less defense/resist and take more damage from the solo player. But the moment a 2nd player enters it's range it will have the normal amount of damage, defense/resist so that players can't exploit it and farm it faster.