If I understand correctly what everyone here is asking for, the objective is to have only one topic (discussion) thread that only members of that group and yourself can see and reply to.
Your course settings -- at course level, not forum settings -- should be set to Groups: Separate groups, Force: No.
As teacher, go into your course and create a new forum with these settings:
Forum type: standard forum for general use
Can participants post...? No discussions, but replies are allowed.
Force everyone to be subscribed? Subscriptions are not allowed.
Group mode: separate groups
As teacher, you WILL have to start a new thread ("Add a new discussion topic") for EACH group.
Since they are separate groups, each student will see ONLY the thread for his/her group. Students can reply to your (teacher) posts and to posts of others in their group. They can not start a new discussion. **This effectively maintains the 'single discussion' forum idea.**
Since it's a 'single discussion' thread, you'll only need to do this once, so no big deal (unless you have oodles of groups).
So, as teacher, click on the name of your forum, and choose the first group from the drop-down menu. Write up your post and submit it. (You may want to copy your post so that you can just paste it in to the new topic threads for the other groups.)
Do this for each group.
Voila! When students login, they see only 1 topic (assuming each student belongs to only one group), and they can reply to it. You as teacher can see them all and reply to all the students' replies.
Yes, if you need to tell something to all groups, you will need to post that in each topic. Or use the 'all participants' if you don't want/need them to reply to that post. Remember, they can still respond to what you post in their group topic thread, they just can't directly reply to your 'all participants' post.
Let me clarify confusion about force groups. In the course settings, all that 'force groups' means is that whatever you've selected--none, separate, or visible-- will be the default that shows up in all activities you add to your course.
If your course settings are Groups: Separate, Force: Yes, then you can not change an activity's (such as a forum) settings to Groups: Visible.
If your course settings are Groups: Separate, Force: No, then you can change an activity's settings to Groups: Visible, if you like.
Let me clarify confusion about Force subscription. Subscription is the feature that sends ALL posts from a forum to ALL course users. It does not recognize groups. ALL posts in a forum go to ALL course users, always. There is NO way for a group to receive via email ONLY the posts of others in his/her group.
This means that forums must be checked manually be students, since they will not be receiving email notification of a new forum post.
So, if you are using Separate Groups in a Forum, set Force everyone to be subscribed to 'Subscriptions not allowed'.