If you’re trying to add a sub-user acount and see the error, “Account already exists”, it means that the email address you’re trying to add, has been registered as an account already.
What likely happened is that this user went to the Reftab home page and registered their own account. They will need to delete their account first from within the settings page, and then you can add them as a sub-account again.
The user needs to login, click “Settings” > “Delete Account“
Once they delete, you can add them as sub-user under your account. At that time, they will get an email in their inbox (make sure to have the check spam if they do not see it) where they can set their password and login.