Joining a team is a great way to stay on top of what's happening in that team and receive real-time notifications for messages posted to the team. Whether you can join a particular team depends on your membership in the organization and the privacy level of that team.
Guests can only view content that was specifically shared with them so joining teams is strictly by invitation only.
Members, on the other hand, have more flexibility. Members can view and join any Open Team, they can view and request to join any Protected team, but they must be invited to a Private Team.
Members can view the list of Teams available to join in two ways: