Group cannot be controlled by a field in another group.

Why group cannot be controlled by a field in another group ?
Is there any reason to this ?

This can be done by commenting lines 484 and 489 like this :

//    if (!$in_group) {
      if ($allowed_values[$field['field_name']] = content_allowed_values($field)) {
        $available_fields[$field['field_name']] = $field;
      }
//  }

You need too modify line 491 changing $context to 'group' (Warning on preg_match).