The following code added to the bottom of P2’s functions.php will allow P2 to send email notifications to all other responders in a comments thread.

// PluginBuddy Email Notification Patch for P2
// by Dustin Bolton on September 10, 2010.
// http://pluginbuddy.com http://site2.zirch.com
add_action( 'comment_post', 'email_notification' );
function email_notification( $comment_id ) {
	$emails = array();
	$comment = get_comment( $comment_id, ARRAY_A );
	
	// Get emails of all responders.
	$comments = get_comments( 'post_id=' . $comment['comment_post_ID'] );
	foreach( $comments as $this_comment ) {
		if ( $this_comment->comment_author_email != $comment['comment_author_email'] ) { // Only add emails that are not this comment poster.
			array_push( $emails, $this_comment->comment_author_email );
		}
	}
	
	$emails = array_unique( $emails ); // Strip all duplicate email addresses.
	
	// Get email address of thread starter to strip them from receiving replies since that is automatic in P2.
	$original_post = get_post( $comment['comment_post_ID'], ARRAY_A );
	$original_poster = get_userdata( $original_post['post_author'] );
	$emails = array_diff( $emails, array( $original_poster->user_email ) ); // Remove from array.
	
	// Send emails.
	foreach( $emails as $this_email ) {
		wp_mail( $this_email, 'P2 thread updated!', "There has been an update to a thread you posted in by " . $comment['comment_author'] . " on " . $comment['comment_date'] . ":\n\n" . $comment['comment_content'] );
	}
}
// End PluginBuddy Email Notification Patch.