User receives "does not exist in tenant" error

Issue: A user attempts to log in and is presented with this message on EVERY computer. Other users can log in successfully on the same computer. 

The user has been set up in CASTiNET, and the user's account is visible in the Azure portal. 

'{username}' from identity provider 'https://sts.windows.net/47bxxxxx-5xxx-4xxx-bxxx-6xxxxxxxx/' does not exist in tenant 'CASTiNET' and cannot access the application '05xxxxx-f8xx-45xx-axxx-e1xxxxxx' in that tenant. The account needs to be added as an external user in the tenant first. Sign out and sign in again with a different Azure Active Directory user account.

Solution: The user has not accepted the invitation from CASTiNET they should have received when their CASTiNET username was created. 

Resolution 1: The user should locate the invitation email and accept it. 

Resolution 2: Contact support and have the invitation resent from the Azure portal.