There are multiple invite mods including the built in one, but as far as I can tell none of them are setup to allow people to earn credits/points. If by chance I'm wrong here, can someone let me know which one specifically.
We have just released an addon - XenCentral Invite System, which has just the basic features for the first release, but we are going to develop it into a fully featured invite system, that will include award manager (not only credits/points, but also user upgrades, integration with other third-party credit systems etc.), friend inviter from social and email services and much more. We are updating the resource with new features, just subscribe to it to be notified when the features you need are developed.
P.S. Another version of the addon allows paid invites and registraiton, and includes (and will include) all features for Invite System. You can check it here if interested.