userid intergrating into link

so here is the problem i am having.

I am making a site with a reward system that would track if people have bought something from an affiliate website, the problem i am having is, i am using buddypress for registration and user accounts. However using the affiliate links they have said if i can add user id’s then i can track who has bought what by using the user id/usernames then i can reward the appropriate person.

Can you help