You can send e-mail to multiple people by seperating the email addresses with a comma.
e.g
$to = 'nobody@example.com'.','; //take note that a comma is used for seperation
$to .= 'nobody2@example.com';
...
if(mail( $to, $subject, $message, $headers )) {
echo 'mail sent succesfully';
} else {
echo 'mail was not sent';
}
It's always good to get some error handling in here. mail() returns true if it was successful and false otherwise so you have a good opportunity for error handling.
Check out the php manual as it has the example I've just explained at.
http://uk.php.net/manual/en/function.mail.php
Hope this helps you out.