You can join a team when the team captain has invited you to the team. The team Captain needs to invite players to the team on a tournament-by-tournament basis so the invitation must be accepted at the beginning of each league season/tournament.
If the invitation is from a team you are familiar with, you can press ACCEPT. You cannot leave a team whose request you have accepted (except by asking the captain to remove you - depending on tournament rules in question), so it is important that y