Group B is the second group who participated in the Trials.
The Gladers (also known as Group A) discovered that they were not the only test group when they met Aris Jones at the beginning of The Scorch Trials. Aris told them that he had been part of Group B. It was composed almost entirely of girls, with Aris being the only boy. They escaped from the Maze three days faster and lost fewer people than the Gladers, showing that they are fairly capable.
The leaders were Harriet and Sonya (until Teresa joined them in the Scorch). Several of the girls' roles seem to be equivalent to those of the Gladers, and the experiences of the two groups in the Maze appear to have been extremely similar. One major difference was that Rachel (Thomas's equivalent) was killed by Gally's equivalent in Group B, but with the Gladers, Chuck was forced by WICKED to jump in the way of the knife Gally threw at Thomas.
Before Group B began the Scorch Trials, they were told by Janson that they must kill Thomas if they wished to receive the cure when they arrived at the Safe Haven. They were allowed to travel through underground tunnels through the Scorch, while the Gladers were forced to travel overland in the burning sun. After they ambushed the Gladers, took Thomas, and dragged him up into the mountain pass, Thomas convinced them not to kill him. Group B and the Gladers met up at the Safe Haven, and they fought together against the Bulb Monsters before being rescued by WICKED.