Solution for “AADSTS90072: User Account from identity provider does not exist in tenant and cannot access application” error in SharePoint Online

Problem: A guest user in SharePoint Online is getting this error message when trying to access a SharePoint Online site “AADSTS90072: User Account ‘User@Domain.com’ from identity provider ‘https://sts.windows.net/…’ does not exist in tenant ‘Tenant-Name’ and cannot access the application in that tenant. The account needs to be added as an external user...