Ask HN: Site Invitation System

My apologies if this has been previously posted about. I tried looking (google as well) but couldn't find anything satisfactory so thought I'd try you guys.

I'm looking to build an invitation system into a site of mine and wondering if any of you knew of a good solution already out there (preferably in PHP, but any language will do for inspiration and guidance). I'd prefer not to have to entirely build my own but will if I need to.

Essentially, I would like a way to easily send invitations to new users, allow existing users to send invites, keep track of said invitations/referrals and also allow for registration codes to be used in bulk numbers to allow for limited user account registration.

Is there an already existent solution out there that I could integrate or use as a blueprint? Any suggestions would be much appreciated.