Just click on the Tables list button:
The Tables list let you see many tables already created and many players sitting at their seats.

To sit at a table, just click on an empty seat and immediatly your profile image will appear there. As soon as all the seats at the chosen table are taken, the match begins.
If you change your mind and want to quit the table, just click again on the seat with your image; the seat will be free again.

Enjoy yourselves!