Yes, I found it extremely helpful too. You can only RSVP if you have the invite code. If you don't then you can't RSVP.
You can store the code in a particular box in the sheet and then your google script can validate the code entered by the guest.
You can store the code in a particular box in the sheet and then your google script can validate the code entered by the guest.
However, due to timing constraints, I couldn't do this and instead used a simple encryption algorithm to have the hacker at least put in some effort to know the invite code :)
I will raise an issue on github for this.