From ABC's official press release:

For the first time the doctors of Seattle Grace and Oceanside Wellness will team up – to save the life of one of their own – on ABC's Grey's Anatomy.

Special guest stars from Private Practice include Kate Walsh, Audra McDonald and Taye Diggs.  Also guest starring Grant Show (Melrose Place, Swingtown) as Addison's brother, Archer, and Golden Globe-nominated actress Melissa George (In Treatment, Alias) as Sadie.

Addison, Naomi and Sam rush the ailing Archer to Seattle Grace and reunite with old friends Derek and Mark to help save his life; meanwhile, Izzie employs her fellow doctors in a medical game designed to help teach the interns, and Owen runs into a significant someone from his past, on Grey's Anatomy, Thursday, February 12 (9:00-10:02pm, ET) on the ABC Television Network

Grey's Anatomy stars Ellen Pompeo as Meredith Grey, Patrick Dempsey as Derek Shepherd, Sandra Oh as Cristina Yang, Katherine Heigl as Isobel "Izzie" Stevens, Justin Chambers as Alex Karev, T.R. Knight as George O'Malley, Chandra Wilson as Miranda Bailey, James Pickens, Jr. as Richard Webber, Sara Ramirez as Callie Torres, Eric Dane as Mark Sloan, Chyler Leigh as Lexie Grey, and Kevin McKidd as Owen Hunt.


Source: ABC
(Image courtesy of ABC)