1. Select Settings on the forum you want change.
2. Scroll down to Common module settings and expand the menu. Turn Group Mode on to Separate Groups and then click Save Changes.
3. If you have not made any groups yet, please follow this article on how to create groups in Moodle, Moodle - Groups.
4. Create a forum in the target week. Once you are in the settings, scroll down to Common Module Settings and expand it. Make sure the Group Mode says Separate Groups, and then Save Changes at the bottom of the page.
5. After you have both groups set up and the forum set up correctly, when you view the forum, you can now see the groups in the drop down menu and choose a new discussion for each group.
   

To have a forum where the students can only reply in their group(s):

1. Go to the participants tab. Click on it and select Permissions from the top left dropdown menu. This is a way to enable the students to only respond to the group forum without letting them create a new discussion thread.
2. Once in Permissions, select Advanced role override. This is a drop down menu, please select Student. This will redirect you to a new page about the override permissions of the student forum.
3. Scroll down till you see Start new discussion. It is automatically set to allow. That should be changed to Prevent on the options, and will only let the students in the group reply to initial forum and not create a new one.
4. Go to the bottom of the page and select Save changes.
5. Congrats! The students will now only be able to respond to the forum question. They won't be able to select Add a new discussion topic. You now have the full authority to add the discussion topic (this is a question that the students should respond too).