The "Folksmen" was the mock folk group Shearer, McKean, and Guest put together...I heard a story where they had them "open up" for a Spinal Tap concert, and the crowd was much perturbed...apparently not realizing the joke, that it was actually the Spinal Tap guys. Actually they have a pretty decent sound for a joke group, see "A Mighty Wind".
Christopher Guest is one of my favorite filmmakers. Although calling him a "filmmaker" sounds exceedingly snooty. Comedian behind a camera maybe?