There are a couple way you could do this.

Simplist, if the door opens off and up stage, or on and down stage then you can just put a stage hand behind the door to open and close it. Put a strip of black fabric between the door and the jam on the offstage side so the audiance doesn't see the stage hand through the gap.

If you can't hide a stage hand because of the way the door opens, or in the case of french doors you can do this with rods. For example you can take a thin piece of wood and put a bolt through one end into a hole on the top of the door right at the edge that opens. The rod would have to extend off to an area there a crew person can operate it without being seen.

Dan