The daily deals site buy with me currently has a deal for $10 for $20 of food at the boston burger company in davis square, and they offer a $10 credit for each new user who buys something via a link you give them. So one ought to be able to:

1. existing buy with me user 0 who intends to buy more things in the future (and so doesn't mind an account credit as compensation) invites new user 1
2. new user 1 signs up new user 2
3. new user n signs up new user n+1
4. new user N can't think of anyone else to sign up, starts the buying reverse chain in the next step
5. user N buys deal, emails user n-1 saying they bought it, unsubscribes from buy with me's emails
6. user n gets email, checks that the account credit has come in, buys deal, emails user n-1 saying they bought it, unsubscribes from buy with me's emails
7. prior buy with me user 0 pays user N $10.

Of course, if you have someone who doesn't mind account credits at all, then you don't need this chain, you can just have a central user who pays other people $10 (or something between $10 and $0) to sign up with their link and buy something.

If anyone feels like doing this, my link is this and I'd be open to negotation on where in the $0-$10 range I should be willing to offer for a purchase via that link and the corresponding $10 account credit.