If you want to spontaneously join folks meeting in another room, the polite thing to do is to knock. When you knock, those already in the room hear a knocking sound and see a pop-up notification. Anyone in the room can click the pop-up notification to grant you permission to enter.
To knock, right-click on the room label and select Knock on [room-name].
This is what a knock request looks like to the person in the room. There are a variety of responses to choose from. Selecting "Come on in" brings the requestor's avatar directly into the room.