Just a little under 3 weeks shy of his 15th birthday, George Harrison joined The Quarrymen. He was first friends with Paul McCartney who then introduced him to John Lennon. It was Harrison's guitar playing (particularly his skill at playing the song "Raunchy" note for note) that won over Lennon. Here's one of the songs they recorded as The Quarrymen- "In Spite Of All The Danger", written by George and Paul.
