How do you build a group out of other groups in an INI inventory, and how do you check the result?

Add a section named [parent:children] and list group names (not hosts) in it; ansible-navigator inventory -i inventory -m stdout --graph parent then draws the resulting tree.

[raleigh]
servera.lab.example.com
serverb.lab.example.com

[mountainview]
serverc.lab.example.com

[us:children]
raleigh
mountainview

* Child groups nest by location, while webservers and production cut across the same four hosts. *

The :children suffix tells Ansible that the lines below are group names. The parent group us contains every host of its children, so ansible us --list-hosts returns three hosts: servera, serverb and serverc. Add a host to raleigh and it automatically becomes part of us too, which is why nested groups beat repeating hostnames.

To see the hierarchy, ansible-navigator inventory -i inventory -m stdout --graph us prints:

@us:
  |--@mountainview:
  |  |--serverc.lab.example.com
  |--@raleigh:
  |  |--servera.lab.example.com
  |  |--serverb.lab.example.com

A @ marks a group. Without -m stdout, ansible-navigator opens its interactive text UI instead, where :0 browses groups, :1 browses hosts and :q quits.

Go deeper:

• Grouping groups: parent/child relationships (official docs) — the :children syntax in INI and children: in YAML.
• Ansible Navigator documentation — the inventory subcommand, stdout vs interactive mode.