In the Team select manage team, then select add member.
Type in the email address of your guest.
Teams will suggest adding unrecognised emails as guests.
A guest can do specific things:
Just for info, this what your guest will see when you select “Add”
Guest gets an email and needs to select the “Open Microsoft Teams” link
The guest will be prompted to create an account linked to the email you used to invited them.
They’ll have to add a password
then give away some personal information..
Then verify the email, this is to stop people setting up loads of Teams accounts that you might not want.
Agree to the usual hoopla
Then join the conversation.
Full details of this can be found here: http://blogs.brighton.ac.uk/teachingcontingency/microsoft-teams/external-users-in-microsoft-teams/