$form array:
    form element 1
        attributes
    form element 2
        attributes
    fieldset1
        attributes
    fieldset2
        attributes

$form['date_fieldset'] = array(
        '#type' => 'fieldset',
        '#title' => t('Select Start and End dates'),
        '#collapsible' => TRUE,
//        '#tree' => TRUE,
    );

    $form['date_fieldset']['date_inactive_start'] = array(
        '#title' => t('Start date'),
        '#type' => 'date_popup', // Provided by the date_popup module
        '#date_format' => 'd/m/Y',
        '#date_year_range' => '0:+1',
        '#required' => TRUE,
        '#size' => 8,
        '#default_value' => date('Y-m-d', time()), // Default value must be in 'Y-m-d' format.
    );

    $form['date_fieldset']['date_inactive_end'] = array(
        '#title' => t('End date'),
        '#type' => 'date_popup', // Provided by the date_popup module
        '#date_format' => 'd/m/Y',
        '#date_year_range' => '0:+2',
        '#required' => TRUE,
        '#size' => 8,
        '#default_value' => date('Y-m-d', time()), // Default value must be in 'Y-m-d' format.
    );

function my_module_form_alter(&$form, &$form_state)
{

$form['#pre_render'][] = 'add_to_fieldset';

}

function add_to_fieldset($form) {
    /**