Luna.a, interesting how you brought up "a group full of psychopaths." I want to mention the twin, she's pretty psycho. However, shows nowadays don't follow exactly what the book tells, and the show may offer twists to the viewers. But we'll see. Also I still think Melissa and Jenna has something to do with this A Team... They too seem psycho in their own way.
Obviously, Mona is A. She mentioned the "A Team" to Spencer in the car (from tonight's episode). I think Melissa and Jenna are affiliated with this whole thing as well. I am pretty sure Melissa or Jenna, either one of them, started this A Team.
Theory, Melissa wanted revenge against Spencer? Jenna wanted revenge against Allison, of course. Mona wanted revenge against the girls because of popularity.