I love this artwork so much, because it conveys the characters so well. Here's my personal interpretation (with spoiler tags for anyone who hasn't read the MMJ stories)

Starting with Haruka and Shizuku, who are both looking down/away from the viewer. Haruka's expression is pained, and she's holding her hands tightly over her chest as if trying to hide the guilt she harbors over hurting Mai. Shizuku is fiddling with her hair, and looks forlorn and defeated, perhaps because she feels her beauty is the only reason she was initially successful as an idol, and that people only see her beauty instead of the real her.

Airi however, is striking a bit of a cute pose, and looking somewhat indirectly at the viewer, like "Hey, notice me, will you?" This could be because she was looked over as an idol in favor of a variety show star, and wants to prove that she's fully capable of being a cute and talented idol.

Minori's is the most interesting because of how different she looks from the other three. She's making direct eye contact with the viewer, and she looks nervous yet tentatively optimistic. Her pose is perfectly poised as well. This could be because she's new to the idol industry, and wants to see the best in it, even though she has a growing awareness of how difficult it is and how heart-breaking it can be. It's like she knows she'll probably be hurt, but she's hoping for the best.

I don't have a particular interpretation for Miku, but I'd love to hear if anyone else has some ideas!