Video conferences can only be started via the Sdui app, within the Sdui chat, by the group admin. These are mostly teachers. To find out who the group admin is, go to the group's member list. 

To do this, open the corresponding chat and click in the header on "Click here for group details".

Here you land directly on the member overview page. The members of the group who are admins are indicated by the "Admin" next to their name. Only these members can start a video conference in the chat.