When updating a field that does not exist yet you need to use the field key rather than the field name.
When dealing with a group field, it can only have one row so I would bypass the loop and add_row and to
$value = array(
// sub field key/value pairs
'field_12345' => 'value 1',
'field_34567' => 'value 2',
// etc....
);
update_field('field_xyz123', $value, $post_id);