Rids was on the right track.
3rd item down on this page:
http://www.amember.com/integration.php
"Different usergroups support - YES"
And if for some reason that won't work, your needs can definitely be met by throwing vbulletin integration into the mix and setting your amember supscriptions to tie into vbulletin usergroups. I know this because I'm doing the exact same thing with my set-up.
Yes, amember can protect physical directories if that is what is needed (using either php sessions or htaccess control) but it does a heck of a lot more with the available third-party integrations.
Hope this helps!
Cheers,
~Regs.