At NPR, reporter Jeff Lunden says:
I've been singing in real choirs since I was a kid, so I was intrigued to participate in a virtual one. I recently asked Whitacre how he came up with the concept.Listen to the interview. Whitacre ended up putting together a choir online of people singing his composition. According to the article, "Thursday night, he unveil[ed] his Virtual Choir 2.0 on YouTube. It features more than 2,000 singers from around the world, including this reporter."
"Well, it all started with this video — a young girl named Britlin Losee, who was 17 at the time, posted to YouTube a video of herself singing the soprano part to a piece of mine called 'Sleep,' " Whitacre says.
The video above is the one referenced at NPR. The video below is Eric Whitacre's Virtual Choir - 'Lux Aurumque' from 2010. You can watch the video that gave instructions to participants here with Whitacre directing.