Facebook has made a few long-awaited additions to Messenger lately, including that new Snapchat-like camera interface. Well, there's one more big change coming in 2016—group video chat. This feature it rolling out to Android and iOS starting this very day. You'll have to forgive the iPhone screens, that's all Facebook has provided. It should look similar on Android.
Messenger has had video calling before, of course. The difference now is that you can have more than two people involved with a video chat. As many as 50 people can join a single video chat, but the UI has to adapt a bit. With six or fewer, everyone is visible on the screen at all times. When you have more, only the "dominant" speaker will be shown on the screen. So, I guess that just means you have to shout over everyone. Yep, that's Facebook.
You need the latest version of Messenger for the feature to work, but probably not a new APK. Facebook usually adds features via server-side switches. To start a group video chat, simply open up an existing group text chat or start a new one. There will be a video button in the upper right to switch over. Everyone in the conversation will be notified and given the opportunity to join via video. You can also directly ring people to get them to join. The rollout is only starting today, so give it some time if the feature is not showing up for you yet.