This is a list of characters from Group B. These characters refrained from joining the first group for different reasons.
Bowser Jr. was ordered to stay behind and formed Group B when he lost contact with his father.
Banjo-Kazooie, Mewtwo, Lucario, Deoxys and Jigglypuff couldn't be found for the froming of Group A, but could be found for Group B.
Lucas and Chunky Kong didn't join Group A because they were too afraid, but joined Group B because they were riddled with guilt over their friends.
Shadow, King Dedede, Little Mac and Meowth declined from joining Group A for selfish reasons, but like Lucas and Chunky Kong, were riddled with guilt over their missing friends.
Team Star Wolf, Wario, Waluigi and Samurai Goroh declined citing that there was nothing in it for them, but joined Group B because Bowser Jr. promised them a reward.
Bio Rex had declined from joining Group A for the same reasons as Team Star Wolf, Wario, Waluigi and Samurai Goroh, but joined Group B as Bowser Jr. had promised him a lifetime supply of Mammoth Ribs.
Beastman had declined from joining Group A so he could keep an eye on Bio Rex, who didn't join Group A. When Beastman heard that Bio Rex had joined Group B, Beastman joined to help keep Bio Rex in check.