Before the official meeting begins, start the team meeting.
Select the participants icon the select the “Copy Join Info” icon.
Paste the Join Info into an email or instant messaging application and send it to the guest.
The guest selects the link in the email and can join the teams meeting.
Guests added to the meeting this way won’t be able to see the chat or access any teams files, but they can use their webcam, microphone and share their desktop/window/slides.
Full details on this here: http://blogs.brighton.ac.uk/teachingcontingency/microsoft-teams/external-users-in-microsoft-teams/