Hi Frances, we have the same problem and I wrote an issue about that in the Moodle Tracker. https://tracker.moodle.org/browse/MDL-41677
We resolved it as follows:
- a general forum for each group
- settings: "available for group members only"; separate groups
- choose the forum
- choose "show/edit current subscribers"
- turn editing on
- add the users you want to be subscribers
there is an old thread about the issue https://moodle.org/mod/forum/discuss.php?d=26282 including:
"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'.
In my opinion it should work in another way as I discribed in the tracker/simon