The Alliance (or The B-Team) is a group consisting of Jenna Marshall, Sara Harvey, and Noel Kahn, and formerly Shana Fring and Melissa Hastings. Their goal was to protect The Liars or to hurt them depending on the situation. The group began in "UnmAsked", but it didn't last long. It dissolved in "A DAngerous GAme".
Four seasons later in "Along Comes Mary", the group appeared to have made a return with Jenna as the leader and Sara and Noel being part of it. However, Noel betrayed the girls by killing Sara and had planned on killing Jenna to get what he hoped was money Charlotte DiLaurentis had left them, but he is killed by the Liars while trying to kill them too. After that, it seemingly dissolved again.
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.