Seperating Forum

[Adam81]

I run a subscription service and I use amember (www.amember.com) as my CMS. Amember relays to vbulletin paid members which gives them access to the forums or automatically makes them a "registerd user". Basically I want my forums to be seperated into a private area only available to paying members and the rest open to any person who registers through vbulletin. I want people to be able to sign up through vbulletin to access the free portion of the forums. I know I can create a new usergroup such as "registerd but not paid" but I don't know how to make that the usergroup you are put in when you register through vbulletin and not amember. SO very basically how do I change the usergroup you are put in if your signed up through vbulletin?