Problems sharing application with others

I have been having major difficulties sharing my Creator database with others. I go in to share, enter their email addresses to invite them, and everything looks fine on my end. I can see their email addresses in the list of users I am sharing with just fine. Unfortunately, they are getting errors on their end.

Even after accepting the invitation, they get a message that "You are not authorised to access this application in Zoho Creator. Go to Home."

I have tried removing them from the shared list and reinviting them, but that hasn't worked either. They have tried logging in both with their Zoho usernames and with their email addresses; neither account works.

Any suggestions on what else we could try to get the sharing permissions working correctly?