Better layout

The list of potential subscribers can get rather long (see this 'before' image).
If the fieldset had an id, it could be targeted with the following CSS rule to collapse the long list into several columns (see 'after' image):

#subscribed-box .form-item { 
  display: inline-block; width:100px; margin:0; 
}

One way to insert the id is to modify the support_subscribe_form function as follows:

function support_subscribe_form(&$form_state, $edit, $title) {
[...]
    $form['subscribe']['subscribed'] = array(
      '#type' => 'fieldset',
      '#title' => t('Subscribed'),
      '#collapsible' => TRUE,
      '#collapsed' => TRUE,
      '#prefix' => '<div id="subscribed-box">',
      '#suffix' => '</div>',
    );

The new #prefix and #suffix lines wrap the fieldset, so it can be identified within CSS