Comedy legend Christopher Guest has signed on to join the ensemble cast of Night at the Museum: Battle of the Smithsonian, according to The Hollywood Reporter. Guest joins Ben Stiller, Robin Williams, Owen Wilson and Ricky Gervais in the follow-up to the 2006 smash hit, which grossed $571 worldwide. Guest will play Ivan the Terrible for returning director Shawn Levy. The film is currently shooting in Vancouver and it's pegged for a May 22, 2009 release date.