To join a tournament, go to the Games section in the left sidebar menu and select Tournaments.
This will show the current tournaments in progress. For each tournament, you can click ‘View this Tournament’ to follow the tournament, chat, games, and standings.
If the tournament has not yet started you will have a link to ‘Join this tournament’. If it's underway, you can Latejoin the Tournament and if it's too late to Join, it will display "Too late to join".