Guest User Error
You can add external users or guests to your Microsoft Teams team. These individuals do not need to have an Elon email address.
Sometimes an error may occur in the form of, "User name [user's email] does not exist in tenant Elon University and cannot access the application in that tenant. The account needs to be added as an external user in the tenant first." As a work around, access Teams through Google Chrome. In most cases, the error is resolved and access is granted after the guest account is recognized by Microsoft's systems.
If you were added as an guest user in another organizations, switch tenants in Teams to view and access all organizations of which you are a member.