Also, Auditor in LLBLGen are plumbing classes that let you define your own auditing code. That means you can get the UserId in the best way you perform your authentication.
See the Auditing exmple (available in download section in LLBLGen site), that example just pick up the user from the session. Maybe you want to use IPrincipal, or you could set the userId to a custom property on your Auditor class, whatever you want/need. It's up to you