Click on the upper-left icon next to your team name.

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/5f4b580c-6bb6-4d8c-8b14-793db3d6b84e/teeam.png

There you will a list of teams that you are a member of.

There will also be an input where you can paste the name of a team and if that team exists you will become a member of that team.

https://s3-us-west-2.amazonaws.com/secure.notion-static.com/ca1264ff-a02e-4f22-b855-d4ead7d81a34/Screenshot_2020-08-14_at_14.55.11.png

The name of a team is exactly the same as what you see in the upper left corner.