Are there any adult sponsors that do this?
 

Say you have a website where people have to sign up to get free access. In order to sign up for your site, they must sign up to the sponsor for a FREE (no credit card info) membership to get access to your site.

Does any sponsor do this? Anyone paying $1 or more for every free membership? Of course, standard protections apply: proxy check, duplicate email checking, duplicate IP checking, etc etc.