Maybe you could set something similar to what ebates.com has done? Each user has an individual login and everytime they go to an affiliate site they are provide

Maybe you could set something similar to what ebates.com has done? Each user has an individual login and everytime they go to an affiliate site they are provided with a tracking ticket. It's a really cool system and I've made over a few hundred dollars with them related to business trips for my "real job". If your site ever starts marketing towards fundraising groups, a user could be asked to enter a simple identifier code to tell the system that all proceeds should be directed to XX cause OR the group could be provided their own sign up link which automatically associates the user with XX cause.