You can execute commands in the form.yml.erb with backticks like
output_from_thing = `/bin/bash do_this_thing`
But I’d advise against it just because it could fail or timeout or any number of other bad things could go wrong and prevent the app from working.
The default for form items is text field so simply remove all the items in attributes and keep the entry in the form section and it’ll be a text field.
Or you could pre populate the list, maybe based on unix groups. Or maybe you could dump the AGs into a json or csv file and read that file in the form.yml.erb. That may be safer than executing commands but I’d warn against reading off of network filesystems becuase if you have an outage on them - well this app is now in the blast radius. So if you go the route of periodically dumping AGs into a file that this app then reads - use local storage and be sure to account for cases when you can’t read the file or you can’t parse it correctly.