Hi @acesuares
AFAIK there is no way to authenticate using ODK Collect directly.
A possible workaround is creating a question asking for a password and use constraint like suggested here So a password for each user, that means you then have to keep track of users and manage the passwords.
You will still have the problem that they then can share their password and the QR code, but at least if you see the same password used with different deviceids, you can know who shared a password
Another option is to create custom QR codes and setting username, in this case you'll then have to be creating QR codes instead of managing passwords. Again they can share the custom QR but you will be able to spot by checking the deviceids.