Which wordpress plugin could help me create an "invite-a-friend" system where the inviter earns commissions on all future purchases by the new user?

Hi all,

This is what I want to achieve.

BUYERS MAY ALSO BECOME AFFILIATES: Using a form on the site buyers may invite friends (by creating account for them and sending them a request to join). When these friends accept, then for all future purchases by each of them the inviter should get a fixed commission (incrementing with more every customer friend that is invited).  Note: I still haven't found a single plugin on the web that can do just this. I don't mind if it s money or points that are awarded to the inviter, as long as they are able to redeem them against future purchases.

I haven't found any such exact plugins. Does anyone know one?
NOTE: Please do not suggest plugins that work on coupon basis or click-a-link or banner basis. I need people to be able to invite friends using a form and then earn commissions on all their future purchases. Note: I have written to the owner of Affiliates Enterprise to see if his would do it even though what I need is not listed on their site. I see that this is compatible with Affliates Gravity Forms with which I could create affiliate account for the invited users. Any good?

NOTE: Commercial themes and plugins are fine.

Please feel free to suggest themes, methods/approaches, plugins or any other means of achieving this. It is an urgent question so all help will be appreciated and for that I thank you in advance.