Well, the question is: How do you know who is invited?
What qualifies an "invited" user?
An easy approch would be to have a "send invitation" script accessible for your members, which would take the email-address of a new member, generate a random invitationid and send a link to the reg-from (including this id) to the email-address.
The script register.php must then check if there is an invitationid and if it is valid, otherwise it must reject registration
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