The great thing about this service besides being free is that invited participants can join the conversation even if they have no Facebook account. The service works online and there is nothing to download or install. You can easily create a room from Facebook or the Messenger app (click on People then 'create a room') then invite others to join. For invitees with no Facebook account you can share with them a generated link. As the admin of the room you can adjust the settings of your room the way you like: you can make it private, select who can view and join it, remove people from a call, or even lock your room. When you lock your room no one else can join it. You will also be able to see rooms that others create if they are open to you. Check this page to learn more about how Messenger Rooms handles privacy issues.
Note that Messenger Rooms is still rolling out and is not yet available everywhere. Watch the video below to learn more about this new service.