Conversation so far:
what is a charity John Mayer works for?
John Mayer has participated in benefits for the Elton John AIDS Foundation.
what does the Elton John AIDS Foundation do?
Supports innovative HIV prevention, education programs, direct care and support services to people living with HIV.
how does the Elton John AIDS Foundation raise money?
The organization supports its work through proceeds from special events, cause-related marketing projects, and voluntary contributions from individuals, corporations and foundations.
did the Elton John AIDS Foundation earn a lot of money?
It has raised over $400 million to support HIV related programs in fifty-five countries.
what is a type of philanthropy John Mayer works with?
In 2002, John Mayer created the Back to You Fund.
does John Mayer work with any other artists for charity?
In response to the Virginia Tech massacre, John Mayer (along with Dave Matthews Band, Phil Vassar, and Nas) performed a free concert at Virginia Tech's Lane Stadium.

Question: how many people attended?
Self-contained rewrite: how many people attended the free concert that was held in response to the Virginia Tech shooting?