He is referring to https://github.com/dwyl/learn-to-send-email-via-google-scrip...

It would be quite easy to check the rsvp code in appscript. E.g. have a sheet with all the guests, and give a random number (rsvp code) to each guest. Then on each rsvp request you could check whether that rsvp code exists in the code column. If yes: you'll add the answer to that row. If no: return an error.