Simply try to keep going with their stories. Some scenes require you to visit their rooms during a specific time of day so keep that in mind as well.
A third guest will show up once Ashley's friendship level is at a certain point. After that, when you buy the second floor, another character will be introduced shortly. Then three more will be added eventually depending on how many days have passed.
Just take it day by day and see where you go. That's the fun of games like this after all, not knowing what will come.