Mona's Army is a group of people who were bullied by Alison DiLaurentis prior to her disappearance and are trying to stop her from rising to popularity again. Mona's army and the A-Team are different because the A-Team wants Ali to stay, but Mona's Army wants her to leave.
Mona calls a meeting to form the group. Paige McCullers is among the recruits but declines to join.
They stand behind Mona's story when a video of Alison slapping her is shown.
Mona came to meet her army in a coffee shop. No one but Lucas Gottesman was there because Alison got to them.