You can open or close the stage door by ending the action taking place on the stage. The stage is then in essence a closed set. Keeping the door closed allows the Erasmatron to maintain clear records of which actors have what knowledge. If an actor is allowed onto a stage in the middle of an action, then that actor's knowledge, of the action, is incomplete and faulty. If there is no action taking place on the stage, then the stage door is always open.
Any actor can choose to enter or leave.
When a story is allowed to run itself without a human player, then the storyengine opens and closes the stage door according to this rule as well.