The Animorphs were a team of mostly Earthlings who used Andalite morphing technology to defend planet Earth from conquest by the Yeerk Empire. The original members were Earthlings Jake, Marco, Rachel, Cassie, and Tobias after they took a shortcut home through an abandoned construction site and were given the morphing power by Tobias’s (then unknown) father Elfangor-Sirinial-Shamtul, using the morphing cube.
They soon received a distress call from Elfangor's brother Aximili-Esgarouth-Isthill, who had been trapped underwater, and after rescuing him he joined their ranks.
Much later an untrustworthy boy named David joined, but he was disposed of by Rachel.
Much much later the Animorphs started recruiting disabled children, after much hesitation, to augment their numbers and possibly heal some of the children in the process. It worked, but with many casualties.