https://www.theverge.com/2018/2/21/17035494/facebook-messenger-audio-video-call-add-friends-family

Facebook Messenger now allows you to add more friends and family members to your audio and video chats while they’re still in progress. With a new feature introduced today, you can add people to an ongoing audio or video call while you’re still talking, rather than having to hang up and manually add people into a conversation before restarting the call.

To add someone while you’re on a video or audio chat, tap on the screen and select the new “add person” icon, then tap on the people you want to add. After you’ve hung up, you can message everyone in the conversation in an automatically created group chat in your Facebook inbox.