facebook group post graph api not allowing me to post

Question

I am trying to post to one of my group but getting below error

Graph returned an error: (#200) If posting to a group, requires app being installed in the group, and \ either publish_to_groups permission with user token, or both manage_pages \ and publish_pages permission with page token; If posting to a page, \ requires both manage_pages and publish_pages as an admin with \ sufficient administrative permission

using below code

$page_token = "";
$group_id = "1258454427550445";

try {
        $publish = $fb->post('/'.$group_id.'/feed',array('message'=>$_POST['status'] .'   via phplift.net demos'),$page_token);
        } catch(\Facebook\Exceptions\FacebookResponseException $e) {
            // When Graph returns an error
            echo 'Graph returned an error: ' . $e->getMessage();
            exit;
          } catch(\Facebook\Exceptions\FacebookSDKException $e) {
            // When validation fails or other local issues
            echo 'Facebook SDK returned an error: ' . $e->getMessage();
            exit;
          }

        echo 'Status updated.<br>';

its saying on this page:

App Installation

Group admins can locate and install your app as described in the Help Center, How do I add an app.

I am following all steps but unable to find my app on my group to add please guide me how i can add my app to my group.