If a team is public, you can request to join a team by clicking on the kebab menu on the top right of the team page, and select "Request to join." If your request is approved, you will receive a notification when you've officially been added to that team.
Alternatively, if you would like to receive notifications for a public team you're not a part of, you can follow it.
You cannot request to join a private team; you must be invited to a private team.