David Crosby met Graham Nash of the British Invasion band the Hollies when the Byrds toured England in 1966; when the Hollies came to L.A. in 1968, they reunited their acquaintance.  At a party given by Nash's then-girlfriend Joni Mitchell in July 1968, Stephen Stills and David Crosby performed a new Stills song called "You Don't Have to Cry" with harmony vocals added by Graham Nash.  The three realized that they had a unique chemistry, and Crosby, Stills & Nash was born. 

(April 2014)