Facebook group chat is constantly under progress feature by Facebook. Here is how you can use the feature.
By default the group chat icon is available for the phones with support for camera. For those with only messenger service with no front camera permission enabled in Android, may not be visible in the list.
1. Open the Facebook Messenger
2. Create a new group. Add only the participants you think will join.
3. You can add upto 50 people.
4. After adding all the users, click on Video tab.
This should start the facebook group video. In this case you have to see that every participants has answered the group call request. Otherwise only those with the accepted requests are seen in the video chat.
Dominant speaker is shown at the middle and rest of the speaker are shown based on their voice input.
Missing the join request is shown up in missed call option.
Group Chat Limit : As per the Verge report, the Facebook allows upto 50 people to get into the group chat. You can read more about it here.
There is also camera change being done here. And the video calling feature now gets option of recording the video like snapchat and instagram like story. Currently limited to only iOS users. This may get released to android users shortly.