When a new club match starts, all members of that club will receive a message in their inbox asking them to join.
You can click the link in the message to view and then join the match. Or, you can go to your club's homepage and look under 'Upcoming Events' where you will see a list of any active/available Club Matches and their details, as well as the number of players who have joined each side.
Click the green 'Join' button to join the match! Or, click the match name to open the match and see the info, such as the rules and who has already joined. Then if you want to, you can click on 'join' from inside the match:
