Hi Dan, currently, the Chained Selects add-on is limited to static population via a CSV file - or - dynamic population via the gform_chained_selects_input_choices filter. If you want to pull a dynamic list of users, you’ll need to go with the filter approach. The filter documentation has some solid examples of how to populate choices once you’ve fetched the desired data.
If you’re looking for a UI-based approach, Gravity Wiz (my own company) has a powerful new add-on coming very soon called GF Populate Anything. This plugin will allow you to chain multiple selects together in a very similar fashion with the added bonus of automatically fetching the data for you based on configurable filters right in the field settings.
Here’s a little sneak peek of how it works.
If you’re a Gravity Perks customer, we’ll be happy to provide you early access to GF Populate Anything via the support form.