Library/project for making an invitation-only website?

[Dani]

Hi.

I want to create a website where new users need to be invited by an existing user in order to open an account.  Think of it the way Orkut became popular.

I need a library, or code from some project, that sends the invitations, keeps track of which user invited who, etc.

I'm sure there's already some code out there that performs these tasks.  Drupy maybe?

Could anybody recommend me some library/project for these tasks?

Thank you in advance.