Picking a list via user role is custom work, but is possible using custom SQL and Formidable hooks to populate your search record. Everything else is done with the User Registration Add-on.

Appreciate the answer Victor -  Thanks very much
Could you please comment on this code?
add_filter('frm_setup_new_fields_vars', 'populate_dropdown_with_users_on_form_load', 10, 2);
function populate_dropdown_with_users_on_form_load($values, $field) {
    // Check if the field is the dropdown field you want to populate
    if ($field->type == 'select' && $field->id == 'dropdown_field_id') {
        // Get all user IDs and names
        $users = get_users();
        
        // Prepare choices array
        $choices = array();
        
        foreach ($users as $user) {
            $choices[$user->ID] = $user->display_name;
        }
        
        // Modify the field configuration with the updated choices
        $values['field_options']['choices'] = $choices;
    }
Is this getting anything like it needs to be?
Thanks for any tips or references
Kind regards 
Mike