Conversation so far:
What did he do in London Underground?
became the most popular musical group of the "London Underground" psychedelic music scene.
How did they become the most popular musical group?
In 1966, a new rock concert venue, the UFO (pronounced as "you-foe"), opened in London and quickly became a haven for British psychedelic music.

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