Hook for URL admin/people/create
/**
 * Implements hook_form_alter().
 */
function mymodule_form_alter(&$form, &$form_state, $form_id) {
  switch($form_id) {
    case "user_register_form" :
      //Your Logic
    break;
  }
}


But for performance issue use hook_form_FORM_ID_alter()