When you are in a chat room where people are sharing their web cam video you will see a panel of video view windows to the left of the group participant list. You can tell who is publishing their video by the camera icon beneath their name to the right. If you are currently viewing someone you would see a green 'eye' icon instead.
Between the text chat area and the video viewer panel there is a 'drag bar'. If you hover your mouse over this bar a tool-tip will appear and you can click on and hold your left mouse button to drag the bar around. When you do this the video viewer windows will re-size and re-orient themselves for the best possible view.
The number of video windows that you can view simultaneously depends on your subscription level.