Generate and send invitation codes to users

Hi guys. I need your precious help. I’m working hard on organizing a contest linked to a wordpress site. That’s the way it should work:
1. There’s a contest. The user have to register as new user in WP (there’s also the Facebook Connect) to enter the contest.
2. When a new user register himself, the system sends him an email: “Congratulations! You are a new user. Here there are some invitation codes (or links) to invite your friends”.

How can i do this workflow? I need to generate some codes (or links) every time a new user enter the WP site and store these codes linking them to the users. I need to know if a new user (called B) comes to my site thanks to another user (called A), in order to give to A a gift.

Thank you guys.