code: def link_user(self, enterprise_customer, user_email):
          
          try:
              existing_user = User.objects.get(email=user_email)
              self.get_or_create(enterprise_customer=enterprise_customer, user_id=existing_user.id)
          except User.DoesNotExist:
              PendingEnterpriseCustomerUser.objects.get_or_create(enterprise_customer=enterprise_customer,
                                                                  user_email=user_email)
docs: Link user email to Enterprise Customer.
  
          If :class:`django.contrib.auth.models.User` instance with specified email does not exist,
          :class:`.PendingEnterpriseCustomerUser` instance is created instead.