Lovato is best known for her work on TV's "Sonny with a Chance" and "Camp Rock," and Ross for her iconic role on the classic sitcom "Happy Days."
The actresses are to appear on the May 13 episode of "Grey's."
"An elderly patient admitted into the E.R. for a heart condition sees a familiar face, a long lost love who happens to be in the E.R. as well for a fractured arm, and the staff find themselves caught up in their love story," ABC said in a synopsis of the episode. "Meanwhile, Karev treats a troubled teenage patient whose parents brought her in for schizophrenia, and Meredith can't help but tell Cristina about her suspicions of Owen -- which inevitably messes with Cristina's head."
"Grey's Anatomy" stars Ellen Pompeo as Meredith, Sandra Oh as Cristina, Justin Chambers as Karev and Kevin McKidd as Owen.