I think the best solution here would be to simply have the system notify every participant by default, and add a "Disable notifications from this comment thread" button/option. Either that or – maybe more preferably – the other way around, i.e notifications from everyone would be disabled by default (as it is now), while the button/option would be "Enable all notifications from this comment thread".
Edit: I'm not sure if people are legitimately failing to understand the premise here or just choosing to stop reading after the first sentence and throw in their downvote. I sincerely struggle to imagine a feature to opt-in to all comment notifications for a particular thread being this objectionable.