External Authorized Account Settings
 

Not sure if the title is elaborate enough.

What I am looking to do is create something similar to what vbulletin.org and vbulletin.com provide.. (Priority Support)

I have a paying user. They input support email/account name for forums in their member area.
On forums it then checks this setting and if they are a paying customer allows access to certain areas/forums... if not it restricts the account to a standard account.
Sort of a "dynamic" user group.

Where should I begin? Is there a plugin/product for this? Just trying to clarify thoughts and see what others think and see if its already been added or not here....