Go to conversation. Now click on the three dots near your own profile.
Go to settings >System > enable group chat
At the moment group chat does not allow calls. But this will change next / in a few months: Url: https://review.jami.net/q/status:open+-is:wip (topic: call-swarm)
At the moment you can make group video calls with 1:1 text chat only OR group chat with no audio/video calls.
Tutorial: https://docs.jami.net/general/faq.html#can-i-make-group-conference-calls
BTW: You need to wait 30 seconds before you make a call (it’s actually bug). And if you make calls (different networks) you probably always need TURN.