Hello,
No, the students in the grouping can write a reply to the forum introduction (which should not be possible), and which of course results in that everybody (within the grouping) can see the comment, all groups (within the grouping) can see all comments. Student can choose a group to make a comment but it makes no difference. Usually a forum which is set to groups is devided automatically into a forum for each group but that doesn't happen here.
From the teacher perspective: The teacher chooses a group to see only the descussion within a certain group but he always sees all discussions - just like when he chooses to see the discussions from all participants. If he chooses a group and writes a comment, all the groups (within the grouping) can see it.
Kristbjörg