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 where the person or people you want to interact with are located and select Knock on [room-name].
In the scenario below, Nate knocked on the "Guests" room where Carrie is located.
This is what the knock request looks like to Carrie:
To allow Nate to join her in the room, Carrie simply clicks the pop-up toast notification in the lower left corner of the map and Nate is allowed in.
Note: "Knock" notifications remain visible for 5 minutes; then, if unanswered, expire. A badge is added to the room chat along with an activity time stamped message.