My site has different memberships, both free and paid (Pro). Our affiliates only get rewards when the invitee becomes a Pro member.

Everyone gets an affiliate link when they become a member. Free members get 1 Credit (awarded residually) to them for each person they invite that becomes a Pro member and Pro members get rewarded with 2 Credits.

So our paid members actually make more from the affiliate system to reward them for being a Pro member. But this also keeps us from getting a bunch of free fake members, hopefully, because there is no incentive for it.