It means you somehow have to develop them all, with depth and realism, through the eyes of a single viewpoint. Yes, that's usually harder. If you can't do that kind of character development in first person, then work with third person till you improve your skills. Experiment with the first and second person in short pieces, exercises, or an experimental book. Third person is more popular, certainly.
Second person is least popular.
However, it's how well you tell the story, not which viewpoint you choose, which is going to make the difference. This depends again on your skill level as a writer, in the type of story you want to tell, and on the tone you want to set. Then pick third person. Consider first person. This may be as simple as including a few lines about the people left in the room after the main character exits.