You can only be on one team at a time, so to join a team, you would need to leave your own team.
There are teams that advertise to recruit members. I see you've found the board, and posted a question, but no one can answer on that board. It is just for recruiting.
You may also see a team that catches your eye, as you play games. If it is a public team, you can just click to join from the team's profile page... again, just one team at a time. You cannot belong to more than one team. The system will not allow it.
All the Chat boards are open, and can be found just below these Forums. The subjects are quite varied, or you can start a conversation.