By default I think he wants everyone to be subscribed to new replies.
Be aware this can put a big load on the mail server if you get a lot of users or a long topic and also you will be forcing them to have an email they didn't ask for which people don't usually like so they will probably turn it off after a couple of emails.
This info is old but I believe still applicable. Back up the file before you do it.
There is this too
https://www.phpbb.com/customise/db/exte ... own_topic/