Just think "group" is a special field. The only difference is it has fields
parameters, which is an array of fields. This is a sample code:
$meta_box = array(
’title’ => ’Test’,
'fields' => array(
// Group
array(
'id' => 'group_id',
’type’= > 'group',
// Subfields
'fields' => array(
array(
'id' => ’text',
’type’ => ’text’,
’name’ => ’Text’,
),
// Other subfields go here
),
),
),
);