Drupal add fieldset inside form and wrap all form elements under that fieldset
It should be something like this.

function change_password_form($form, &amp;$form_state) {
  if (user_is_anonymous()) {
    return $form; // Or drupal_access_denied()?
  }

  $form['#account'] = $GLOBALS['user']; 

  // To make the fieldset define #type as fieldset
  $form['fieldset'] = array(
    '#type' =&gt; 'fieldset',
    '#title' =&gt; t('Password Info'),
    '#collapsible' =&gt; TRUE, // Added
    '#collapsed' =&gt; FALSE,  // Added
  );

  $form['fieldset']['current_pass'] = array(
    '#type' =&gt; 'password',
    '#title' =&gt; t('Current password'),
    '#size' =&gt; 25,
    '#required' =&gt; TRUE,
  );

  return $form;
}