Paid Memberships Pro Affiliates Addon GPL Overview:

The Lightweight Affiliate Tracking AddOn for Paid Memberships Pro creates unique affiliate codes assigned to your affiliate users. When someone visits your site using an affiliate URL (a code is passed to a page as a parameter), the affiliate’s cookie is set for a specified number of days. If a cookie containing an affiliate referral code is present after checkout, the order is logged into the affiliate’s account.

Affiliates can see a report of their activity by creating a page with the shortcode pmpro_affiliates_report (optional). The admin can view a report of affiliate activity in the admin under Memberships > Affiliates as well as add and manage affiliate codes.