Guests can't enter certain forums

[lilah]

I would like to have to kinds of forums for my mybb:

one which guests can enter, read all topics and post their questions in
and another one that guest can't enter. but they should be able to see the forum on the index (no topics or subforums) so they can tell that there are other forums and there is activity in these forums.

so far I found no configuration and no plugin that made this possible. I would be very grateful if someone could help me with that.
thanks in advance