What is the threat model? Do the organisers have an incentive to cheat? Do invitees have one?
If the organisers can be trusted then the solution is to have a device displaying a QR code that rotates regularly (30 seconds?) at the location and have attendees scan it.
If the invitees can be trusted then the process can be reversed. Each invitee displays an unique QR code during check-in the organiser should scan.
If neither can be trusted then you’d need a neutral third-party to act as a witness by scanning both the organiser’s code and the invitees’. This proves that the witness was at the event (and it was indeed open to the public) and that invitees did show up.
Out of curiosity, how do you prevent a malicious party from creating dozens of fake accounts and using them to check in at their fake event?
