The recent posts button can help with this in a pinch, as it will show you all the recent posts. If your site doesn't have too many messages per day, you can just open
http://site.com/thread/recent and all the new ones are there.
But more technically it sounds like what you want is automatic subscriptions for every post...
In this thread I tried to find the right area of code for the default setting for the subscription button, checked or unchecked, however I may have been wrong and there has been no resolution on this issue. It could be helpful for you to know something about where certain settings are.
http://forums.modx.com/thread/95866/change-defaults-for-thread-subscribe
If I may ask, what do you want? When a user makes a new post, you want notification sent to you and others, right? And when a user replies to a thread you want to be notified as well...do I have this right?
I *believe* moderators assigned to one board will be notified of new messages on that board, but I might be wrong about that, as I haven't used board moderators. And I am a little unsure what the real default settings are for notifications.
Do you have any notifications working at all so far? Do I have your goals defined correctly? Sorry I can't give you a straightforward answer...