To my way of thinking the proper way would be to create 2 forums (with or without Global Masks) solely for the groups use, making one for regular group users/members in which all the members of the group could post and read topics. And a second forum where only privileged, users/members of the same group could read and post topics.
This would simply require that the users choose the proper forum when deciding to post a topic.
Assign a group leader (or more) to add members to the groups as desired/needed, assign some Moderators and you are all set. That's pretty much how the forums are designed. You could even have a 3rd with (global masks) for all registered and anonymous users.
You can also assign posting passwords to the forum but that would be only a partial answer to your question. And I am quite sure there are other, more elegant ways to accomplish what you want.
Here are some links that should help confuse you, some more:
But that is not what I understood that you were asking; I may have misunderstood what you want to do, or how you envision it but it sounded to me like you want a group in which virtually every member is a limited group leader of sorts, able to set allowed permissions specific to one topic, and to my mind that would not work on scales of magnitude I can only begin to imagine or explain!
And to the best of my knowledge the only way to actually do that without coding it would be to literally create a forum for each user and let them be their own group leader and I don't think that is a very good idea.
Yet I have seen forums do just that (passing them off as personal Blogs), but I have never seen them work out so well.
NOTE: You said you were a "Noob" and I take you at your word, but do not take what I say as the absolute gospel, I am an amateur as well, but I'm certain that if I have misstated something here someone will correct me.
I am still working on an explanation of the permission system as I understand it, that I was drafting for another project (its still not quite finished, but if you need more help understanding what you want to do I will make it available) but I am afraid it might just confuse you.
If you have any questions just ask and I will try and help as best I can.
I hope this helps.