Task: Question Answering
Input (English): Passage: Jo Elizabeth Stafford (November 12, 1917 – July 16, 2008) was an American traditional pop music singer and occasional actress, whose career spanned five decades from the late 1930s to the early 1980s. Admired for the purity of her voice, she originally underwent classical training to become an opera singer before following a career in popular music, and by 1955 had achieved more worldwide record sales than any other female artist. Her 1952 song "You Belong to Me" topped the charts in the United States and United Kingdom, the record becoming the first by a female artist to reach number one on the U.K. Singles Chart.
Born in Coalinga, California, Stafford made her first musical appearance at age twelve. While still at high school she joined her two older sisters to form a vocal trio named The Stafford Sisters, who found moderate success on radio and in film. In 1938, while the sisters were part of the cast of Twentieth Century Fox's production of Alexander's Ragtime Band, Stafford met the future members of The Pied Pipers and became the group's lead singer. Bandleader Tommy Dorsey hired them in 1939 to perform back-up vocals for his orchestra.
In addition to her recordings with the Pied Pipers, Stafford featured in solo performances for Dorsey. After leaving the group in 1944, she recorded a series of pop standards for Capitol Records and Columbia Records. Many of her recordings were backed by the orchestra of Paul Weston. She also performed duets with Gordon MacRae and Frankie Laine. Her work with the United Service Organizations (USO) giving concerts for soldiers during World War II earned her the nickname "G.I. Jo". Starting in 1945, Stafford was a regular host of the National Broadcasting Company (NBC) radio series The Chesterfield Supper Club and later appeared in television specials—including two series called The Jo Stafford Show, in 1954 in the U.S. and in 1961 in the U.K. 
Question: Who did Tommy Dorsey hire to perform back-up vocals?
Output (English): The Pied Pipers.