Referral System for Membership Plugin: Members reffering Members

Hi,

I am looking for a plugin that will allow me to combine a simple referral system with Membership. Its not really an Affiliate plugin that I'm looking for. I just want new users to be able to input an email address when they register of the person who referred them. When someone refers X amount of people then they would get lifetime membership for free. Its more similar to the system on here where you get free membership for posting 1000 times.

I suppose it fits under the category of gamification more that affiliate. Does anyone know of a plugin like this or is this something that might one day be done for WPMU dev? I think a lot of people running membership sites would benefit from this or a more comprehensive gamification plugin.

Thanks in advance.

Paul