To initiate a group call, you simply have to tap on the phone icon in a group conversation. After initiating the group call, you can manage the individual participants. If you miss the group call and it is still in progress, then you can tap on the Phone icon within the group chat to join.
Unfortunately, Facebook Messenger does not support group video chats yet. If you want to have a group video chat, then it is recommended that you use Skype or Google Hangouts instead. Facebook will likely launch group video chats at some point to stay competitive against other messaging apps. Facebook Messenger currently has 900 million monthly active users and many of them are clamoring for a group video chat feature.
During the group call, the Messenger app will display the group name and the list of the participating users. Users that are invited to the call can accept or decline it. This feature can be useful for calling multiple family members, friends and business contacts at the same time.
Facebook Messenger rolled out free Wi-Fi calls for users back in April 2014. WhatsApp, the mobile messaging app company Facebook acquired for $19 billion, integrated free voice calling for Android and iOS last year. And Facebook Messenger added the one-on-one video calling feature for users about a year ago.