[MoMan]

I don't think there's a hack out for this, but it sounds pretty easy to code up if you have a comma-separated field in the forum editor. Here's some pseudo-code that you can put wherever the current e-mailing calls are made:

$usergroups = '(' . $usergroup_field . ')'; // you probably want to validate input for your actual implementation
$emails = $vbulletin->db->query_read("SELECT email FROM _vbuser WHERE usergroupid IN $usergroups");

while($result = $vbulletin->db->fetch_row($emails))
{
vbmail($result['email'],$title, $message);
}