You can find the players who are at any table you see in the Lobby, and you can find the players who are at a table you have joined.
In the Lobby you see active tables in either the Open Tables tab or the All Tables tab. Click on a table in the Lobby and that table's details appear in the Information pane on the right side. Here you'll find those currently seated to play as well as the observers. To display a player’s information, click on the player name.
After you have joined a table either to watch or play, you can easily
find out who else is at your table. In the Information pane, click the
Players tab, then click the At Table tab (if not already selected). This
shows all players at the table (those watching or playing). Watchers are
distinguished by an identifying icon 
at the top right
of the player picture.
If there are 20 or fewer players at the table, the players' pictures are shown. If there are more than 20 players, a text list view is shown.
When the players' pictures are displayed, you can see summary details about a specific player by moving your mouse over the player's picture. To fully display a player’s information, click on the player's picture (or name in the text view list).