code: def register_instances(self, load_balancer_name, instances):
          
          params = {'LoadBalancerName' : load_balancer_name}
          self.build_list_params(params, instances,
                                 'Instances.member.%d.InstanceId')
          return self.get_list('RegisterInstancesWithLoadBalancer',
                               params, [('member', InstanceInfo)])
docs: Add new Instances to an existing Load Balancer.
  
          :type load_balancer_name: string
          :param load_balancer_name: The name of the Load Balancer
  
          :type instances: List of strings
          :param instances: The instance ID's of the EC2 instances to add.
  
          :rtype: List of strings
          :return: An updated list of instances for this Load Balancer.