Prerequisite for starting video conferences: You are the group admin. Most of these are teachers. 

To start a video conference, go to "Chats" in the footer and either select an existing chat or create a new one in which you want to start the video conference.

Click on the red highlighted plus in the footer of the chat window. Here you can choose "Video Conference".

Now enter a conference name and click on "Start video conference".

Now (only) the members of the chat in which you started the conference can participate in the conference. The other participants now only have to click the "Participate" button and are then directly in your video conference.

Note: By default, participants have 60 minutes to join the conference.

If the participation is on a computer, no additional program is required. A new window opens and the person is immediately in the video conference.

If the participant takes part in the video conference with a mobile device (such as a cell phone or tablet), the "Sdui Meet" app must be downloaded beforehand. This is available free of charge from the PlayStore / App Store (for more information see "Can I also take part in the video conference with my mobile phone?")