Try this one:
function prefix_get_users(){
$args=array(
'blog_id' => $GLOBALS['blog_id']
);
$all_users=get_users($args);
$arr=array();
foreach ($all_users as $val) {
$arr[]=$val->data->user_nicename;
}
die(json_encode( $arr ));
}
Reference: https://codex.wordpress.org/Function_Reference/get_users