Lovato—who first got famous for her role on Disney's Sonny With a Chance sitcom—will play Dani, a struggling artist in New York and friend/potential rival to Rachel. She'll make her Glee debut as part of a Beatles tribute episode, so starting placing bets on which classic track—"Yesterday"? "Wild Honey Pie"?—she'll cover for season five.
Incidentally, Adam Lambert is also joining Glee this fall for several episodes. And since Fox would never miss a chance to milk this show for all it's worth, that means Lovato and Lambert will "have several musical numbers together." But then again, why front? The idea of those two singing together is pretty damn enticing.