Compatible XF Versions:
Yes, but removable with payment
This is a 'simple' referral system.
By simple, I mean it allows users' to specify a referrer and have users' that a user referred listed on a page.
A referral link based on username is displayed in a users' personal details edit page, and a referrer field is present on the registration form.
This add-on is live at XF Liam for testing - this is my referral URL: https://xf-liam.com/register/?ref=Liam+W (in plain tags to stop auto-link).
This add-on contains a small branding line next to the XenForo branding line separated by a pipe (|) symbol. Branding removal is available at a cost of £35 to remove branding from all of my add-ons from one site.