I've done zero testing on this but my thoughts are that the majority of people who are willing to pay for porn, like to enjoy it in relative anonymity. They may make porn jokes with their buddies but they don't tell their buddies that they pay for porn or what specific paysites they like. Are there exceptions? Sure. But I think what would happen is you'd have 99% of your 4 "bonus users" systematically sign up with their leader for each site they join. And it would probably be online forum buddies who don't even know each other. 1% of the time, you'd actually get more paying members out of it.
