In JFormFieldUser class there are two functions:

• protected function getGroups() { return null; }
• protected function getExcluded() { return null; }

Function getInput() invoke both this methods which is totally pointless because it always assign null values to this variables:

$groups = $this->getGroups();
$excluded = $this->getExcluded();

Then this always null parameters are passed to com_users&view=users&ayout=modal view which is working pretty nice when you provide some values. For example when getGroups function is recoded to:

protected function getGroups()
    {
        if(isset($this->element['filter'])) {
            return explode(',', $this->element['filter']);
        } 
        return null;
    }
``` using filtering field in form file like: 

allows to show only users from particular user groups in modal because this filtering option is already implemented in this view (without hiding filter field).