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Id they are able to chat with you, there will be a camera icon next to their name in iChat. Simply click on the camera icon to start the video.
 
To clarify, make sure there is the "stacked camera icon" next to their name. This implies they are able to video conference. A "single camera icon" means they are only able to video chat with one person at a time.

Also, you need sufficient bandwidth to maintain a conference chat.
 
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