As noticed in #5004 the JFormFieldUser field was ready to accept "groups" and "exclude" filters, but the code was actually never completed.

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).

# Testing Instructions

1) Apply the patch
2) Add a new User in Joomla and place in in a group (like Authors)
3) Open the form file of an existing component (like /administrator/components/com_contact/models/form/contact.xml)
4) Find the field with "type=user" in that file
5) Add an attribute "groups" to that xml field and place a csv list of user groups ids
6) Add an attribute "exclude" to that xml field and fill it with a csv list of user ids you want to exclude
7) Try creating a new contact from com_contact and check if selecting the user you see only the users in the "groups" and that are not "excluded"