You could create a small plugin to perform the redirect. Here's something I've quickly mocked up:
class plgSystemMYPLUGIN extends JPlugin
{
public function onUserAfterLogin()
{
$app = JFactory::getApplication();
if ($app->isAdmin())
{
$user = JFactory::getUser();
$groups = $user->get('groups');
$groups_to_redirect = array('13', '10');
if (in_array($groups[0], $groups_to_redirect))
{
$url = JRoute::_('index.php?option=com_hellowworld');
$app->redirect($url);
}
}
}
}
I've used 10
and 13
as example user group ID's, so you would have to change them to whatever suits your needs.
Please also note I haven't tested this, but hope it points you in the right direction