As a player, you belong automatically to a guild. If you haven't joined one yourself yet, you are actually from the assassin guild, but it's highly recommended that you join one yourself. This section will give you some more info, if you don't know what guild to join. If you want to know more, you can always ask a player of that guild. Also, every guild has a GM and mostly one or two Enforcers. The GM, the Guild Master, is the leader of the guild, and the Enforcers are his or her "assistants". They can surely help you if that means recruiting new members.
Note: Fish Speaker will be added soon, if I have the info I need. Oh, and the time.

Fremen
 IMG (ixian)
 Bene Gesserit
 Harkonnen
 Tleilax
 Warrior
 Assassin
Atreid