The A-Team

The "A-Team" may be a secret group of people who use the psuedonym "A" to threaten the liars Aria Montgomery, Hanna Marin, Spencer Hastings, Emily Fields, and Alison DiLaurentis while she was still alive. Mona is the only known member of the "A-Team" thus far, as revealed in "UnmAsked."

When Mona Vanderwaal was revealed to be "A," she tried to convince Spencer Hastings to join, what she referred to as, the "A-Team." She also alludes to it when she says "You had to earn it, the right to be a part of it." After Mona was admitted into the Radley Sanitarium, we hear in her mind that everything is going exactly as they planned. A mysterious person in a red jacket visited her and Mona told her that she did everything she told her to do.

Mona Vanderwaal
To date the only known identity of "A" is Mona Vanderwaal. She is possibly working for an anonymous character that wears a long red trench coat. However, it is revealed at the end of the episode that Mona could simply be a worker for the stranger in the coat. It seems impossible that Mona is the only "A" because she is seen getting a text from "A" while in Hanna's room, though nothing is absolutely clear. So she could to be aided by someone, possibly the red coated stranger. Because of the alliance it is still uncertain if Mona is the original "A" or if "A" recruited her later in the series to pose as "A" in some instances. (ex. Threatening Anne Sullivan, following Spencer to the motel). This would make sense since Mona's motive was that the girls were stealing Hanna away from her, and the girls started getting text from "A" before the 4 girls got close again.

Trivia

 * In "UnmAsked," Jenna, Lucas and the Black Swan are seen together. Considering the events of the Season 2 season finale, the Black Swan may be a member, which suggests that Jenna and Lucas are also part of the "A-Team."
 * It's further speculated that Jenna Marshall is on the A-team because during "UnmAsked," she meets an unseen person in the park and hands them an unknown item with the instructions "you know what to do." Some believe this person to be the same person who visits Mona at the end. This might make sense if Mona and Jenna formed a friendship after Halloween 2008, as alluded to in "The First Secret." This would might also mean Noel is on the A-Team, since he had been helping Jenna and the whole relationship and breakup with Mona was staged.
 * It is also possible the term "A-Team" is simply a figure of speech used by Mona while trying to convince Spencer to join. The existence of the "A-Team" has not been concretely established on the show.
 * In "Blind Dates," "A" sends Emily a box of Danby merchandise and handwritten note which reads "SO HAPPY TO HAVE YOU ON OUR A -TEAM!"