Conversation so far:
What did Syd Barrett do in London Underground?
During the London Undergroud period of Pink Floyd, Syd Barrett wrote most of the songs for Pink Floyd's first album.
How did Pink Floyd become the most popular musical group at the rock music venue UFO?
Pink Floyd, the UFO house band, after making appearances at the rival Roundhouse, became the most popular musical group of the London Underground psychedelic music scene.

Question: What did they have to do with Syd Barrett?
Self-contained rewrite: What did Pink Floyd have to do with Syd Barrett?