Hi Chris,

You'll want to use the DotNetnuke.Security.Roles.RoleController class to add roles to your newly-created user; there are several AddUserRole methods hanging off of this class, one of which should suit your purposes.

From your description, however, it sounds as if you are using the MembershipProvider.CreateUser method to create new users; instead, consider using the CreateUser method of the UserController class.  In addition to (eventually) delegating control to your custom provider, this method will automatically add the "auto-assigned" roles to the user (in addition to performing other business-related functionality, such as caching).  In this way you may likely avoid a lot of the role-addition work with which you're currently burdened.

Hope this helps!

Brandon