Add a Guest Collaborator with Can Seat access and they'll receive an email invitation to collaborate on the Event that includes a password to login to the Retired Check-in App on iOS.
The collaborators can then log into the Retired Check-in App on their iOS device, using their email and the included password.
If you received a code that doesn't seem to be working, try resetting your password by going to socialtables.com/login and selecting Forgot Password. Even if you don't have a Social Tables account of your own, you can always reset your password via email. If that doesn't do the trick, please contact Social Tables Support at firstname.lastname@example.org.