If you created a room and want to change it from private to open, or vice versa, you can do so by going to your mural settings.
Here's how to change a room's settings:
Go to the room you'd like to modify.
Click the arrow next to the room's name.
Click Room settings from the drop-down menu next to the room name.
Click Room Settings.
From there, you can change your room's name, type (private or open), and description.
*Note: Private rooms are available only to those who have been invited in; open rooms are available to everyone in the workspace.