Great little extension!
However, lots of invites will end in spam folders because the extensions sends invites from the logged in user's e-mail address. If this address has SPF rules set up not allowing the wiki server to send mails (which in my world would almost always be the case), the receiving server will mark the message as spam.
Shouldn't this extension use $wgPasswordSender as From address?
Anyway, thanks for the great work!
Best regards, Asbjørn Baagø