Getting UserId of Created User

The app requires user (mainUser) to login.  Once logged in, the user can create users (subUser).  I use the CreateUserWizard Control for this.  I need to get the UserID of the subUser.  HOw can I do this.  I'm not sure of how to do it without confusing ASP of which userId I want.  Is this a case for sessions?  Or is there a more direct way?
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