Hi Rosie,
Let me try to answer as good as possible
1. A) You can create separate categories/forums.
B) The entire board can be hidden for non-members. Or only specific forums.
C) Different levels of access to forums can be given to a certain 'member status' or individual members (on username basis). There is however, no
default option to create 'user groups' and assign permissions to those groups.
I don't know difficult it would be to realize that. There are lots of aspects to think of. Search feature for one thing.
Don't know how dynamic the group of users is. But it might suffice to manually add the users to the ACL of the forums.
2. If a member creates a topic or replies to one, they have an option to receive emails on new replies to that topic.
There is no default option to receive emails on every new topic or reply, in a forum. Pretty sure that would be categorized as SPAM very soon.
But, this is something that we could add for you.
If you would like people to stay updated on the activity going on, they could do so with the "Today's Posts" link..?
All in all, I think XMB could be a candidate for your purpose.
Kind regards, Patrick