According to Marcus, “Really excited to announce the global roll out of group calling in Messenger today. From any group conversations, just tap on the phone icon to initiate a group call. You can then manage individual participants on the next screen. If you have the latest version of Messenger, you should see the phone icon appear in group conversations within the next 24 hours.”
This appears to apply to both the iOS and Android versions of the app, so don’t worry about it arriving on one platform before the other. Users who miss out on a call can easily join in by tapping the phone icon, similar to how Skype calls work. Note that there are some limitations in the sense that you can’t create a group call for your entire Facebook friends network.
According to Facebook who clarified the limitations, the limit on group calls is 50. That’s a pretty huge number and it seems unlikely that many of us would be making such calls, but either way it is there if you need it. This will also require users to have the latest version of the app, so update if you haven’t done so already.