This is exactly what I am looking for, however I don't see the mod listed. How can I access it?
There is no official mod for this. But if you do the forum notification mod, you can then go through and comment out (in the tpl files) the ability to opt out. That will bring you most of the way to where you want to be.
If you want to go the last step, and have the notification only go to specified groups, that's more complex. What I did was to add a column in the groups database for group_notify. If you selected group_notify when editing a given group, it would put a '1' in that column. Then, whenever anyone was added or subtracted from that group, I set things up so that all users in that group would get a '1' in the forum_notify column. Sorry, but I ddin't really keep track of what I did very carefully.