Re: Paid membership and subscription plugins
The most obvious is group subscription/filtering I think. Basically, that someone can pay to get access to a private group. That would be a pretty killer feature.
I also think he should provide some pass through variables in the payment buttons. So, he could add a hidden variable to the buttons for groupid=”1234″ and then after the payment is processed, that ID would be sent to the “welcome page” where you could welcome them to that specific group. You could do the same thing for userid and make that userid a friend once they’ve paid.