Can't get a cluster on AWS

I have two AWS instances running in the same private subnet. I used a bitnami AMI to launch. ES on both instances starts ok, but they do not connect as a cluster. The AMI is advertised as ready to go, but it doesn't have the AWS plugin installed. Its unclear to me if the AWS plugin is required, or if the Zen discovery module is all I need.

If the AWS plugin is required, if there are any links to example elasticsearch.yml with AWS settings, it would be appreciated. Thanks.

If instances can communicate to each others on port 9300 using the private IP address, you don't need to use AWS plugin.