I have a simmilar question...
CASE:
we have 5 memberships:
1. free
2. content A
3. content B
4. content C
5. content D
A Paul registers for free. After awhile i send him an invite code that allows him to get access to content A for free. The code must work only once, which means that if John tries to use it after Paul, it won't work.
Codes can be stored somewhere in the plugin files and i will have them printed on paper. When i want to give Paul the invite code, i will just overline it from my printed list (because after usage it won't be avaliable anymore) and Paul will register for the membership that is offered with that code.
So there should be a set of codes that are connected with content A and the other set of codes connected with content B etc.
Paypal is only emerging in our country and the majoriti of new paid member will be found "on the terrain" giving them a "ticket" with invite codes or sending them a mail with invite codes.
I hope i explained it well :-?
Can it be done?