I am looking for an Invitation System for my site

[BlueBBle]

Anyone know any invitation system that is built for PHP sites? I want to make an invitation system like feedthe.net where people can't access their site unless a member from there send u a code that you can use it to sign up but the code works once. anyhow i can achieve this?

[Invent]

I think we might be using invitations on mine & Caleb's new site, if we do, I'll send you my code (once it's written).