There are a couple of different ways you can find where your friends are playing. Follow these steps to meet up with your friends on the tables:
Friends Online Tab
When you click on 'Friends Online' near the bottom right of the page, it will indicate which friends are currently online and playing, by a green table icon next to their username. Hover over that with your cursor and another box will appear with the table they're playing at below 'Playing now'.
You can go to your friend's profile, click 'Playing now' near Last Seen if they're playing at a table, and then it will show you a link to join the table.
In the lobby, a friend's avatar will appear on the right side of the table they're playing at or tournament they registered for in advance.