From NPR:

Until a video of "Stand by Me" had gone viral on YouTube, Roger Ridley had sung and played guitar anonymously on the streets of Santa Monica, Calif., for years. The video begins with Ridley and then mixes in 40 other musicians from around the world. It's part of a 10-song collection called Playing for Change: Songs Around the World.
...

"I approached him after the performance and said, 'Hey, if I come back in an hour with some recording equipment and cameras, I'd love to record you, film you, add musicians around the world to it,' " Johnson says. "And he looked at me really funny, sort of thought I was crazy. But he said, 'OK, if you come back, we'll do it.' "
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The song always starts with Ridley, then Johnson puts headphones on Grandpa Elliot in New Orleans, then plays those recordings for a tribe of Zuni Indians to add a drum, and on and on for a total of "37 other musicians that never met around the world."