Subscribe member groups to forums

We would like to auto-email our members when a new topic is posted in any of our forums. User groups are assigned to different forums, so, ideally, the notifications would be sent to all members within the groups assigned to each forum. Users should also be able to opt out of the notifications.

Is there an add-on that would allow for automatically subscribing all members of a particular group to a forum? Or, is there a way to assign topic notifications in the forum settings to member groups instead of email addresses? Does EE even have a function to email all members of a particular group?

Thanks for your help.