I just hope they work it like the pen and paper game where you have to multiclass into it after you have met certain requirements. The multiclass system in 3rd edition is one of the best things about it and I really hope it's represented accurately in NWN.
I'm not sure how they'd work in the roleplaying elements of gaining the class. For example, a potential assassin must kill someone for no other reason then to join the guild.
I certainly hope not to see adventuring parties teeming with Artemis Enteri's. He's a very nasty man...
