If you want to join an existing team, go to the team page and click the Join button under the members list:
Once you join a team, you'll be prompted to fill out any standups or check-ins that have been configured for that team.
You'll also receive a digest email that contains submissions from your team members. These can be turned off in your notification settings.
You'll also see a link to the team in the sidebar on your workspace for convenient access to the team page: