The infamous group was actually formed by legendary actor, Humphrey Bogart as a informal "Men's Only" club that got together to get away from their wives, drink and gamble. They met at their various houses in Holmby Hills, California throughout the Fifties. Bogart always refered to the group of the Holmby Hills Rat Pack.
When Bogart died in 1957, sometime member Frank Sinatra took on the mantle of leadership along with good-friend Dean Martin. It was Sinatra and Martin that transformed the group into the hip, happening and hard-partying crowd that has become immortalized in Hollywood legend. The core group (and an ever-changing B-List of hangers-on) played together, worked together and performed together - mostly under the garish lights of Las Vegas - until well into the mid-sixties.