How to show user count for individual blog/site

I need to write a quick function that gets the total number of users for a particular blog within my WPMS install. (The function will go inside my theme's functions.php file).

I found a tutorial that uses a simple SQL statement, but this returns the number of users from the entire WPMS network -- not for a particular blog.

function show_user_count() {
	global $wpdb;
	$users = $wpdb->get_var("SELECT COUNT(ID) FROM $wpdb->users");
	return $users;
}

How can I modify that SQL statement to give me only the users from one blog?