how do i collect all the "app" ips in this pillar list to make a comma-separated string like... 172.31.23.64,1.2.3.4,etc.

gistfile1.txt

hosts:    # production
  app:
    - name: app
      ip: 172.31.23.64
      full_host: foo-bar-baz@saltstack.com

    - name: fakeapp
      ip: 1.2.3.4
      full_host: here-just-to-test-salt-template-iteration@foo.com

probably something like (not tested, tho):

{% for name,ip,full_host in pillar['hosts']['app'].items() %}
my name is {{name}}
{% endfor %}