You can join a teams meeting as an external user without a Teams account or Teams installed.
To join directly via the web you need to have received a meeting invite similar to the one below
Click on the Join Microsoft Teams Meeting Link
You will then get a screen like the following
If you have Teams installed you can Join via the teams client, if not select "Continue on this Browser"
If you have Teams installed but still wish to join via the web you may get the following screen, just select "Join on the web instead"
You will get a pop up asking to allow it to use you camera and micrphone, you will need to Allow this to use them in the meeting
You can then enter your name and click Join Now to enter the meeting
You may enter a lobby, a meeting leader will then allow you to enter the meeting