Is there a way to subscribe to all topics, existing and new, at once in any given forum? For example, theoretically speaking, if I were interested in everything anybody has to say about SilverStripe Forum module, I would like to subscribe to "SilverStripe Forums >> Forum Module" forum. If anybody starts a new topic, I would like to receive an email, so if I knew anything about that topic, I would be able to contribute right away and not find out about the new topic next time I go to forums.
Or am I missing this option someplace deep in the admin interface?
No interface for bulk adding subscriptions in the forum. If you want to subscribe to everything your best bet is to use the range of RSS feeds available - you can subscribe to the whole forum, individual forums or even keywords.
If your root of forums is at your.company.com/forums, then the RSS feed for the whole forum is at your.company.com/forums/rss.
The issue I am running into is the same as WalterW's in this thread. My forums are private and require authentication, both basic for some users and external for others. See Pigeon's suggestion in that thread for how to overcome the authentication obstacle with a random hash. I have not implemented it on my forums, so cannot confirm the success of that solution.