Public rooms can be joined by anyone. Public rooms that you have joined appear in a list on the left hand sidebar under the Rooms header. They are denoted by the chat bubble icon. You can search through a list of all public rooms by clicking on the Rooms header.
This will display public rooms you have already joined, and ones that you can still join. To add a public room, click the '+' icon and select Public for the room type.
You can leave and join a public room any time you wish. For more information on this, see our article on joining and leaving rooms.