How can i know who has created user?
By default Drupal don't save anything about who create a user.
If you want do it, go to user account fields  admin/config/people/accounts/fields add entity_reference with select list widget, then in your custom module in hook_form_alter set default value for field with get current user uid and then hide it there, in this manner the current user id (the user that create user) saved in fields without he/she know that this, also you can dont use hook_form_alter just get $user-&gt;uid(the current user that create user ) form global $user in hook_user_insert

shortly


add a entity reference field to user save creator uid in
hook_form_alter when or hook_user_insert