Right now you can make group voice calls in Facebook Messenger on your mobile phone. Facebook may soon roll out this feature even on your desktop PC. People have been able to make group voice calls in Facebook Messenger on their mobile phones since April 2016.
The group voice calling on desktop PC is gradually being rolled out to a test group. If you want to see whether group voice calling has been made available to you or not, just go to your Facebook messages and go to a group conversation. If you can see the phone icon it means you can make group voice calls on your desktop PC.
Group voice calling has been available in other instant messaging apps like Skype on desktop for a very long time.