How to add relationship at the time of new user creation in drupal?
You could create a Drupal rule to achieve this provided you have that functionality turned on. 

Could use the 'Civicrm contact has been created' event.

Checkout the Drupal 7 rules module: https://www.drupal.org/project/rules
Civicrm comes with Civicrm rule intergration module which you can enabled (although you may not need to enable this).