You are correct. In this example I’m only allowing one field of a particular field group to be modified:
$options = array(
'field_groups' => array('group_55788c04bcfd8'),
'fields' => array('field_55788c13e6bf0'),
);
acf_form($options);
Getting the group key and field keys is easy and generally overlooked by a lot of people. Click on screen options. To see the group key show the post slug. Showing the field keys is a radio button.