Referral URL parameter - Set a custom parameter for referral links.
Track HTTP referrer - Track the address of the page (if any) which referred the user to the referral link.
Use profile page as referral link - The user profile page will become the referral link.
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...