This question has been answered by multiple community members. See the first response.
<?php
/* only operate on new resources */
if ($mode != modSystemEvent::MODE_NEW) return;
$parentId = $resource->get('parent');
$res = $parentId;
/* top level documents are not regarded! */
if ($parentId > "0")
{
$parentResource = $resource->getOne('Parent');
$groups = $parentResource->getMany('ResourceGroupResources');
foreach($groups as $name => $grpObject)
{
$resGrpId = $grpObject->get('document_group');
$success = $resource->joinGroup($resGrpId);
}
}foreach($groups as $group) {
$resGrpId = $group->get('document_group');
$success = $resource->joinGroup($resGrpId);
}