While no one quite knows who’ll be sitting in the judges charis over at “American Idol,” Simon Cowell’s “X Factor” keeps adding mentors. It was announced on Tuesday that Justin Bieber will join the show, mentoring L.A. Reid’s group of contestants. Nick Jonas has already joined the show as a mentor of judge Demi Lovato’s group. Britney Spears and Cowell will be mentoring contestants as well.