Idealy when they subscribe, you would remove them from users and add them to the members group. But mypaymentpal doesn't work that way. It simply adds and removes into a group.
Maybe thats something we can address in future versions.
But that's normally how you would do teasers for member only forums. Just setup all the regular forums to be seen by the "member" group as well, then you'd remove them from users when they are added to members.