https://techcrunch.com/2018/02/21/facebook-messenger-now-lets-you-add-more-people-to-in-progress-calls/

Facebook Messenger now lets you add more people to a voice or video call without hanging up first. The new feature is available with the latest Android and iOS Messenger update.

Previously, users who wanted to turn a one-on-one call into a group call needed to end the call, then go through the process of starting another call. In Messenger, this meant they needed to create a new group chat first or look in their inbox for an existing chat to launch the call from. The new feature saves time by showing users an “add person” icon when they tap the screen during an in-progress call. As with other group calls on Facebook Messenger, after everyone hangs up, they remain part of an automatically created group chat.