Are you using BuddyPress also?
Something like this perhaps?
public function user_listing() {
global $json_api;
$key = null;
extract($json_api->query->get(array('key')));
// optional - you may want to validate a "key" parameter?
// authorise($key) ? die('Access denied') : false;
// Which users do you want?
$args = array('role'=>'subscriber');
$my_users = get_users( $args );
foreach($my_users as $handle => $my_user){
$$my_users[$my_user->ID]= array(
'UserID'=>$my_user->ID,
'UserName'=>$my_user->display_name,
'AvatarImageURL'=>bp_core_fetch_avatar(array('html'=> false,'item_id'=>$my_user->ID,'object'=>'user'))
);
}
return array(
"users" => $my_users,
"message" => ''
);
}