If a user reports getting a blank page when logging in, it's likely because the user is not associated with a team
It may be that you've invited a user to the system but haven't assigned them to a team, or that you belong to an AD organization that allows users to be automatically created.
In both cases, you should check the user's team association and ensure that the user is assigned to the correct team(s).
Here's how to assign a user to a team.