If you tap [Join] on the team info page but can't join a team, these could be the reasons.
• You can't join because it is a private team. For private teams, tapping [Join] sends a join request. You can then join the team, once the team leader accepts your request.
• Your join request to a private team can't be sent because you're still a member of a team.
• You can't join because the team has the max number of 50 members.
• You can't join because you haven't cleared the team's required stage listed in the team info.
• You can't join because you've been kicked from the team in the past.