Complex sentence: Crosby, Stills & Nash (CSN) is a folk rock supergroup made up of David Crosby, Stephen Stills and Graham Nash, and they become Crosby, Stills, Nash & Young (CSNY) when joined by occasional fourth member Neil Young.
Split into simpler sentences: Crosby, Stills & Nash (CSN) is a folk rock supergroup made up of David Crosby, Stephen Stills and Graham Nash. They became Crosby, Stills, Nash & Young (CSNY) when joined by occasional fourth member Neil Young.