Born in Izmir, Turkey, Guest came to prominence in the 1960s as a promoter with Harold Davison, and with Brian Epstein's NEMS enterprises. He oversaw the London concert debuts of Simon and Garfunkel, the Incredible String Band and Fairport Convention. He was also an agent during the early careers of Al Stewart and Sandy Denny. Guest was also co-founder, with Jim Haynes (of the Traverse Theatre), of the Howff folk music club in Edinburgh. Notably Bert Jansch started his career at the club which became a meeting place for folk musicians including Archie Fisher and Owen Hand, and the folk duo of Robin Hall and Jimmie Macgregor.