Additional features, settings, or issues not covered by specific Microsoft Teams categories
Hello philt1980,
Thanks for your detailed post here.
From your description, the guest cannot be added into your team. Given this situation, I suggest you go to Microsoft Teams admin center and add the guest into your team to see if the guest can be added. The premise is that the guest is already included in your Azure AD user list.
Besides, the owner can also try sign into myapps portal and select Groups, then find the group you want to add guests into and add the guest in that page.
If you still get the error, to further assist you, please reproduce the problem and collect the Debug logs, then provide it to me via Private Message for further analysis.
Thanks for your time.
Regards,
Seven