Karen,
As Dr. Bhatia points out in his previous post the first step is to turn groups on in your course settings page. You have the option of the default being "No groups", "Separate groups" or "Visible groups". Separate groups is setup so that students in one group can't see what the other group is doing in any particular activity. Visible groups allows students in one group to edit/post in their activity and read what is happening in the other group. To force the group setting at the course level makes it so that you have no control in configuring each activity independently, all activites will exhibit the behavior of the option selected at the course level. Given the parameters that you list in your post I would recommend that you
not force groups at the course level so you can have the flexibility to set group options on each activity independently.
With the description that I have just given, it is also helpful to understand how groups function in the Moodle environment. I will attempt to describe this in a way that is somewhat understandable. Once you have assigned your students to a group using the "Groups" link in your "Administration" block and you have turned on groups in your course settings page you can begin to create activities that use groups.
Every time you create an activity in Moodle and assign it to a group, Moodle creates an identical activity for each group in your course. So as an example lets say you have 2 groups; Group A and Group B. You add a new forum to your course and turn on groups. Moodle creates a forum for Group A, Group B and another for all participants. You as a teacher will see all 3 forums, Group A members will only see posts from the other members of their group and Group B members will only see the posts from other Group B members, assuming you have turned on separate groups. Everyone will see the forum for all participants.
As a teacher you will need to setup each of the 3 forums to get the posts started. You can do this by selecting the group in which you want to post from the drop down list in the upper left hand corner of the screen.
Given the parameters that you have set above, I think now you just create the activity and tell the different groups which activity you want them to participate in and not configure the activity for all of the other groups. Am I making sense? I am actually working on a groups tutorial for 1.6 to be used in our district. If I remember I will post it up in the
Moodle docs site or link it back to my tutorial site. If you have questions please let me know.