What about the tried-and-true rubberband, handkerchief (whatever) on the doorknob? Tell her that sometimes you guys will want/need some privacy (tell her that this goes for her, too), and that it'd be nice if when she comes home and sees the band, etc, that she hang somewhere for a while. If you tell her that you're willing to do the same for her, she should understand. If not, get a doorlock on the inside.. you know, one of those slidey ones.. then she can't come in anyways. lol
