If you believe they are legit, would it change something if they are in a guild or not?
Everyone has to understand that every month, these people pay a fee in order to keep the server up. So they sacrifice something in order to have this server up.
Their characters being legit, why then would they be forbidden to be in guilds and play like any normal player? They should have more than the right to be in guilds or possess guilds in order to do pvp for fun.
Every server of every kind of online game has GM's. All of them have GM that actually play the game, they just don't ever show their identity to players just to prevent this kind of trouble. You need to have a blind faith in GM, every online game can be cheated no matter which it is. But instead of believing in 'what if they do cheat' why don't you believe in 'what if they don't cheat'.
Its in the human nature to accuse others out of jalousy. But this server belongs to them and they should have the same right as anyone else to be players and guild members.
Personally I don't care if its a GM or not that im facing as long as I can hit em real hard with my sword.