Guild Rules
Mages Guild: To enter the guild just go to one of the several guild representatives in most cities and they will assign you a quest for you to prove your worth in one of the schools of magic. It is very easy to be banned from the guild and very hard to prove your worth to them again
Warrior's Guild: Basically the same as the Mages Guild.
Thieves Guild: This guild is much more subtle about recruiting and to be invited you simply must steal a few items or be thrown in jail and are able to escape. Once than you shall receive an invitation one way or the other and from than just follow the instructions than.
Assassin Guild: Very easily the most.... Special guild, to get an invitation you must kill an innocent person who has done nothing wrong to the face of the law. Once you have done the deed one of the guilds member shall extend an invitation and from there just follow the instructions.