When a Guest (external) user is attempting to access a Team hosted on the UI Office 365 tenant with their org credentials (e.g. external-user@xyz.edu) the user cannot already be signed-in to the Teams client for accessing Teams on their own tenant. 

If already signed-in to the Teams desktop client or web app, the external user may get an error.

If invited external users/Guests are getting errors when accessing your Team, have them try the following:

1. If signed-in to O365 in a browser, sign-out and close
2. If signed-in to the Teams desktop client, sign-out and close
3. Open and click on the email invite to the external (UI) Team
4. Select to use either the Teams desktop client, or the web app, and then follow the sign-in prompts