This is a horrible idea, because it invites abuse. If someone sends spam advertising your site, you'll have to deal with the fallout. You might think that a few

This is a horrible idea, because it invites abuse. If someone sends spam advertising your site, you'll have to deal with the fallout. You might think that a few simple checks will prevent fake members, but a botnet can create fake members and make them log in as many times as the botnet's controller wants. Can you really afford to police a referral system?