How do I turn off notifications (for replies) on a forum for all users (but still let them be a member)?

So there is a setting on our community that makes every registered member, also a member to a specific subgroup (with a forum that they automatically receive notifications from).

I still want everyone to be a member of this subgroup (so they should still be able to see the content), but I do not want everyone to receive notifications for that forum (or at least, not from a reply from that forum).

How do I do that? Turning notifications for forum off in general is not the solution, since some members want to opt in voluntarily. 

Thank you in advance!