This has been happening to me so many times. But its always been happening whenever I have a social event and invite ALL of my friends! Sometimes they leave when the whole family leaves or they have to go to school or a job. But eventually if all of the above doesn't work, I have to resort in locking them in a room and kill them. But when it is one of like the daughters/sons friends I have to wait for my them to go back to school so that if its who I want them to marry or something. :\