Approval by Role

Before I spend too much time on how to achieve such a thing, I have to ask how it is your user registration process is.

By default, I believe all users who register are at whatever level you have set in your WP Admin settings. However, it sounds like you have some variable way to set them from the start. Otherwise, the only way I could see the roles being differentiated is by human intervention and manual registration.

If you could give me details on how it is with your setup, I can try to work something out to help.