If you really want to do this, you can see an example of sending a PM to a group in admin/derevokemod.php. Look for "Now inform all other Admins and mods". Refer to function send_message() in ubbt.inc.php for allowable values of the $Group parameter.
If you wanted to send to a group other than one of those groups, you would need to tweak the code.
The reason I said it was inefficient is because each copy of the PM is stored in the database for every user whom it's sent to.
