To start a video call, right click the contact and click Start video call. It is also possible to add video to a standard voice call by clicking the video button and then clicking Start My Video.
You can have a video call between multiple users. You can see up to five concurrent video sources at once. If there are more than five video streams you will see the five from the people who have most recently spoken. Or, you can pin the video streams you want to keep active by clicking the pin in the top right of that video stream.