Answer by taking a quote from the following article:

Henry John Deutschendorf Jr. (December 31, 1943 - October 12, 1997), known professionally as John Denver, was an American singer-songwriter, record producer, actor, activist, and humanitarian, whose greatest commercial success was as a solo singer. After traveling and living in numerous locations while growing up in his military family, Denver began his music career with folk music groups during the late 1960s. Starting in the 1970s, he was one of the most popular acoustic artists of the decade and one of its best-selling artists. By 1974, he was firmly established as one of America's best-selling performers, and AllMusic has described Denver as "among the most beloved entertainers of his era".

Denver began his recording career with a group that had started as the Chad Mitchell Trio; his distinctive voice can be heard where he sings solo on Violets of Dawn, among other songs. He recorded three albums with the Mitchell Trio, replacing Chad Mitchell himself as high tenor. The group Denver, Boise, and Johnson, which had evolved from the Mitchell Trio, released a single before he moved on to a solo career.  Bill Danoff and Taffy Nivert, credited as co-writers of Denver's song "Take Me Home, Country Roads", were close friends of Denver and his family, appearing as singers and songwriters on many of Denver's albums until they formed the Starland Vocal Band in 1976. The band's albums were released on Denver's Windsong Records (later known as Windstar Records) label. Denver's solo recording contract resulted in part from the recording by Peter, Paul, and Mary of his song "Leaving on a Jet Plane", which became the sole number-1 hit single for the group. Denver recorded songs by Tom Paxton, Eric Andersen, John Prine, David Mallett, and many others in the folk scene. His record company, Windstar, is still an active record label today. Country singer John Berry considers Denver the greatest influence on his own music and has recorded Denver's hit "Annie's Song" with the original arrangement.  Olivia Newton-John, an Australian singer whose across-the-board appeal to pop, MOR, and country audiences in the mid-1970s was similar to Denver's, lent her distinctive backup vocals to Denver's 1975 single "Fly Away"; she performed the song with Denver on his 1975 Rocky Mountain Christmas special. She also covered his "Take Me Home, Country Roads", and had a hit in the United Kingdom (#15 in 1973) and Japan (#6 in a belated 1976 release) with it. In 1976, Denver appeared as a guest star, along with Olivia Newton-John, who made a cameo appearance, on The Carpenters Very First Special, a one-hour TV special broadcast on the ABC television network. A highlight of the program was John singing a duet with Karen Carpenter of a medley of "Comin' Thro' the Rye" and "Good Vibrations", although the medley was never released commercially as a single or on an album.

what other artists did his record company work with
Tom Paxton, Eric Andersen, John Prine, David Mallett, and many others in the folk scene.