how can i inform other users about new user registration? [closed]

You can use the user_register hook and then mail all your subscribers.
Something like:

add_action('user_register','notify_new_member');

function notify_new_member(){
  global $wpdb;
  $usersarray = $wpdb->get_results("SELECT user_email FROM $wpdb->users;");    
  $users = implode(",", $usersarray);
  mail($users, "New Member", 'A new member has just joined.');
}