Hi Joel,
I am afraid Virgil is right. There is no built-in feature which would enable you to subscribe all users to a certain forum post. However, you could do it quite simply through Kentico API while looping through all users. Creating a sample forum subscription looks like this:
// Create a new subscription
ForumSubscriptionInfo fsi = new ForumSubscriptionInfo();
fsi.SubscriptionForumID = ForumContext.CurrentForum.ForumID;
fsi.SubscriptionEmail = HTMLHelper.HTMLEncode(txtEmail.Text.Trim());
fsi.SubscriptionGUID = Guid.NewGuid();
fsi.SubscriptionPostID = ForumContext.CurrentSubscribeThread.PostId;
fsi.SubscriptionUserID = MembershipContext.AuthenticatedUser.UserID;
// Subscribe
ForumSubscriptionInfoProvider.Subscribe(fsi, DateTime.Now, true, true);
More API examples can be found in ~/CMSModules/Forums/Controls folder and a complete Kentico API documentation here - Kentico 8.1 API Reference.