Possibility to restrict registration of a new user conditioned to an invitation to existed user.
a) Existed user lopgins and goes to "Invite a person" and type the mail of new user.
Automatically new user receives new email with the code of invitation and can login with mail, and the password (the code of invitation)
b) One person wants to be new user. He go to "register" and the form has only "Avaliable only for invitation users. Please put your email, name and the reason you want to someone invite you".
The user puts this information and any existed user could see this information and if he/she agrees invites the new user
I think this could be easy to implement. WordPress mu have a similar plugin.