Not sure where to post this so I am posting it here.
I get the impression that a large chunk of appsheet users are organisations where a lot of users from the same domain use the app.
Our case might be a bit unusual in that most of our users are private individuals and have completely different email addresses.
I have come across two issues so far:
When someone has registered their yahoo email address as an Apple ID, authentication through apple does not work. This is probably more an apple problem than an appsheet problem.
When someone has an old firstname.lastname@example.org email address, google will return the googlemail email even if they are logging on as email@example.com. This gives you an error message saying that firstname.lastname@example.org doesn’t have access to the app (even though email@example.com does).
I suspect there isn’t much you can do about problem 1 as it looks like an apple issue.
For problem 2, maybe you could give a user access even if the email returned from google (which you are part of) is the googlemail version rather than the gmail version? I am not an expert of google’s mail domains but my understanding is that the two addresses are one and the same.