Choose a topic or week in your course, and with editing turned on, click on Add an activity... and select Forum from the list.
Give the forum a name and an introduction.
Scroll down to the bottom of the page to the Common module settings box. The Group mode settings gives you three options:
No groups. This is the default. Student's can see everyone's work, regardless of which group they are in.
Separate groups. Students can only see the work of students in their group. As far as they are concerned, they have their own forum.
Visible groups. Students can see the work of students in the other groups but they can only interact with students in their own group. It's rather like splitting a class in two where both groups can see what the other is doing but they can't interfere with their work.
4. When you have made your choice, press the Save and display button.
5. If you have specified one of the two group modes, then you can choose which group you want to...