How do external participants join a Teams meeting?

When you send a Teams meeting invite to third party or external participants, you can also include this helpful link to the official Microsoft guide on how to join a meeting without a Teams account. Note that there is a tab for joining via Desktop, and another tab for joining via Mobile device. 

You can also watch this Microsoft video, which includes the various ways you can join a meeting with or without a Teams account. 

NOTE: You won't be able to see Gallery view in a meeting if you've only joined via web app or mobile app, though you will be able to see one video feed at a time. You will have Gallery view if you download and install the full Teams app.