nodder
n.— «I thought it would be just the two of us and a hi-fi. But I ended up in a recording studio with about 20 nodders; a nodder is someone who gets paid to agree with the person paying him.» —“Cowell Regrets Not Being Honest About Murphy’s Pop Tunes” Contactmusic.com (United Kingdom) Jan. 15, 2007. (source: Double-Tongued Dictionary)
One of P.G. Wodehouse’s Hollywood short stories features a Nodder. (He hopes to work his way up the career ladder to Yes-man.) The story probably dates from the 1930s. If it’s to the point, I can look it up.