Urgent user email hack needed

[Hitterman]

I need one hack which will automatically email all the users in a particular usergroup when a new post or thread is created in a particular specified Forum. Just like the one which emails the mod when a new thread is created in the forum which he is moderating.

I think this is not a tough job.

Please can anyone make this hack?

[Marco van Herwaarden]

Why can they not subscribe themself to the forum if they want to have update emails?