That option already exists.
Go to your Edit Options
section of your Preferences after logging in. Go down to "Default Thread Subscription Mode" and change it to whatever kind of e-mail notification that you would like depending on how often you would like to be notified (instant, daily, or weekly) regarding replies in that thread. Don't forget to click on the "Save Changes" button at the bottom of the page.
You can also subscribe to individual threads by clicking on "Thread Tools" at the top of each thread and selecting "Subscribe to this Thread." The same options apply here as well.
You can unsubscribe from threads by going to your List Subscriptions
page in your Preferences.
Hope that helps,