Check User is a Blog User on Network Site

mdw_user_has_blog_role.php

<?php
/* protects site from those who are not assigned to it */
function mdw_user_has_blog_role() {
	
	/* get the global blog id */
	global $blog_id;
	
	/* check user is part of this blog, and die if they are not */
	if( ! is_blog_user( $blog_id ) )
		wp_die( __("You are trying to access a site that you are not allowed to see. Naughty you!") );

}
add_action( 'init', 'mdw_user_has_blog_role' );
?>