He was a founding member of The Hollies , but left that band to form Crosby , Stills & Nash with David Crosby and Stephen Stills . 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 ) .