Close/Open the Door
By default, the doors to rooms are open. This means anyone can click on the room label of a room that has an open door and enter it.
When you want to have a private meeting, you can close the door. Then, if someone clicks to enter, they will not be allowed in unless you grant them access.
To close the door to a room you are in, right-click on the room label and select Close the Door.
This is what a closed door looks like:
To open a closed door, right-click on the room label and select Open the Door.
This is what an open door looks like: