Before1969/1970, the only acknowleged puppeteers were Jim and Frank, but Jim did have a lot of help. Jerry Nelson pitched in quite a bit before officially joining in 1970. Jim's wife Jane helped out before she decided to stay at home and look after the ever-growing number of Henson kids. Carroll Spinney did a number of Anything Muppets although was only credited with Big Bird and Oscar.
Another puppeteer who was never recognized was a gentleman named Cary Antebi. If you grew up in the New York City area in the early 70's, you'd recognize his puppet work from his characters Sherlock the Squirrel on The Magic Garden, and Rags on The Patchwork Family.
So anyone of the above mentioned could have participated in the pitch reel.