Looking for an Opt-in/Opt-out for the guestbook/comment block

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I am hosting a small readers club area on my site. Only registered users are able to access and they are all able to make comments on a recent book or section that we are all reading or studying together. I am trying to find a way to allow all the members in that study to be able to get a copy of the comment when it is posted. I have revamped the comment block to allow me to send to multiple email addresses but I have to hard code edit all the email addresses.

I am trying to find a way to allow a person when they are logged in to click a box and they will receive a copy of the posted comment. Kind of like the "monitor this discussion" button that this forum uses.