The Alliance (or The B-Team) was a term used to describe a group of individuals who were involved in an unlikely partnership for the sake of a greater goal. The Alliance was originally created by Melissa Hastings around the events of "UnmAsked" to protect her sister from The A-Team. She worked with Shana Fring and, briefly, Nigel Wright; they dissolved in "A DAngerous GAme".
Many years later, Jenna seemingly reconstitutes the group in "Along Comes Mary", this time, joined by Noel Kahn and Sara Harvey. Their goal was to locate the alleged "treasure" that Charlotte DiLaurentis had left behind after her death. Their helpfulness to The Liars seemed to vary, but ultimately, the group turned on each other as Noel murdered Sara and forced Jenna to help him kill the Liars at a school for the blind. Jenna betrayed Noel and the Liars accidentally killed him during their struggle. With two members dead and Jenna left with nothing, the group was defunct.
Series
Season 2
The Alliance first comes to light when Jenna meets Shana (unknown at the time) in the park and gives her instructions.
Season 3
Jenna returns to the park to meet Shana again with similar instructions. Later in the episode Melissa meets with Shana and Jenna at the Cavanaughs' house and shows her the invitation to Spencer's fake party. It is unknown how they are aware of the party as this was a plan made by "A" to get all four of the Liars there. Later Toby is knocked out by someone and the Lodge is set on fire. Many fans believe Jenna did this. At the end of the episode, on the police surveillance camera in Detective Wilden's car, Jenna and Shana are seen helping Wilden up and walking away after Ashley hits him with her car.
Season 4
Melissa tells Spencer that she has been protecting her before 'it' started, hinting that they are against the "A-team". She also tells Spencer that Wilden set the fire and she sent Shana and Jenna to watch her at the lodge (without knowing Shana actually started the fire) and that they were being blackmailed by somebody else, besides Wilden, about the N.A.T. Club and Alison connection.
Season 7
Jenna returns to Rosewood after being away for five years and meets Sara Harvey at The Radley. They talk about how they're both "unfortunate" to have both known The Liars and quickly bond. After spending the day together and becoming friends, Noel Kahn meets up with them at the end of the episode as they all share creepy smiles.