if (!isset($vbulletin->userinfo['userid']) OR $vbulletin->userinfo['userid'] == 0 OR !is_member_of($vbulletin->userinfo,x,y,z)) { print_no_permission(); }
where x,y, and z are usergroups allowed to see it.
You can also put a condition around the faq link in the navbar so only certain usergroups see it.
I haven't tested this, so make sure you test it out after to do it.