Monkey Business also featured actors from All About Eve and The Mating Season (and Gentlemen Prefer Blondes, which I've seen too many times.) Who are they?and no one tried to answer (Scott doesn't even count). Ok, it's like this.
Thelma Ritter was in All About Eve and The Mating Season.
Marilyn Monroe was in All About Eve, Monkey Business, and Gentlemen Prefer Blondes
Charles Coburn was in Monkey Business and Gentlemen Prefer Blondes (he was Piggy)
Larry Keating was in Monkey Business and The Mating Season (and Daddy Long Legs, but I didn't include that one. He also had a long running role on the TV show Mr. Ed).
Turns out Thelma Ritter and Marilyn Monroe were two other movies together -- As Young as You Feel and the infamous The Misfits.