Form displays without  after page renders everyone!

I'm trying to embed custom form (only file upload item, will attach the code below). But I see only inner stuff like divs and inputs. No form tag at all.

In page callback I use function that calls theme. here's hook_menu() and page callback

    $items['invite/%'] = array(
      'page callback' =&gt; 'invite_page_content',
      'page arguments' =&gt; array(1),
      'access callback' =&gt; 'invite_check_access',
      'access arguments' =&gt; array(1),
    );

    $output = array(
      '#theme' =&gt; 'invite_view',
      '#type' =&gt; 'markup',
      '#content' =&gt; array(
        'g_id' =&gt; $g_id,
        'g_name' =&gt; $g_name,
        'user' =&gt; $user,
      ),
    );

    return $output;


In theme file I have call of the form 

    $attachment_form = drupal_get_form('invite_attachment_form');
    echo render($attachment_form);


Here's form function

    function invite_attachment_form($form, &amp;$form_state) {
      $form['attachment'] = array(
        '#type' =&gt; 'managed_file',
        '#name' =&gt; 'upload_attachment',
        '#title' =&gt; t('Attach additional file?'),
        '#upload_location' =&gt; 'public://email-attachments/',
        '#upload_validators' =&gt; array(
          'file_validate_size' =&gt; array(2 * 1048576),
          'file_validate_extensions' =&gt; array(
            'png gif jpg jpeg mp4 ogg avi mov wmv flv ogv webm txt pdf zip rar',
          ),
        ),
      );

      return $form;
    }


Any help appreciated!

UPDATE

I've found what was the reason for such behavior. There is some script in my site that deletes first form element. So if I add  before my form render - everything will be ok. Need to find what is it.
$attachment_form = drupal_get_form('invite_attachment_form');
echo drupal_render($attachment_form); //That should work 


see https://api.drupal.org/api/drupal/includes%21form.inc/function/drupal_get_form/7.x