Registration referral system

As always, great plugin. A question I have requires a man of your caliber 🙂

I am looking for a member referral system. Where a current member refers someone who signs up and enters or selects their username from a list to obtain credits. The system would somehow need to track them for credit purposes. None of the plugins out there do anything like this that I can find.

How could we build something like this into tml? Thanks!