Upgrading Registered users to a private section of the forum

[msotojr]

I've been researching how to do this but I have not found anything concrete.

First off the general forum is free to everyone but there are certain sections where I don't want the free users to have access to unless they purchase a product.

What I want to do is automatically registered users to a private section of a forum but not all.

This is what I mean if a person purchases a product and I want to give them access to a vip section. Is there a way to do this by them using a code in the registration link?

If they already a member to go to as a vip member.

I don't want to use a payment api to do this because the product will be on clickbank most likely

I really appreciate your help
I hope this makes sense.