How to add nodes of a autoscaling group automatically to nginx or HAProxy?

In the above architecture (source: http://ift.tt/1QZrN0r), The application server cluster belong to an autoscaling group. But is load balanced by a software loadbalancer (like nginx or HAProxy). My question is if the how the nodes in autoscaling group registers itself automatically with the loadbalancer(As I understand Elastic loadbalancer has this capability builtinm which may not be case for nginx or HAProxy)