Original: In 1968, after a visit to the US during which he met David Crosby in Laurel Canyon and began recreational drug use, Nash left The Hollies to form a new group with Crosby and Stephen Stills. A threesome at first, Crosby, Stills & Nash later became a foursome with Neil Young: Crosby, Stills, Nash & Young (CSNY).
Simplified: He was a founding member of The Hollies, but left that band to form Crosby, Stills & Nash with David Crosby and Stephen Stills.