Passcode only log in

Has anyone been able to modify the php to allow guests to log in with ONLY the assigned passcode?. I have asked the developer how he feels about using member modified php since after all it is his plugin; he was all for it to help solve my issue in his email back to me. Can anyone help me please my fiancé and I are trying to finalize our wedding invitation set up so we can mail our invitations in the next week or so. Thanks!

https://wordpress.org/plugins/rsvp/