In D7, the following code is straightforward, no surprises.
$form['add_contact'] = array(
'#type' => 'fieldset',
'#title' => t('Add Contact'),
);
$form['add_contact']['add_f_name'] = array(
'#type' => 'textfield',
'#title' => t('First Name'),
'#maxlength' => 50,
);
$form['add_contact']['add_l_name'] = array(
'#type' => 'textfield',
'#title' => t('Last Name'),
'#maxlength' => 50,
);
return $form;
This renders a single column in a nice box.