If you have existing Groups in your site, you can make question topics available to specific Groups only. 

When creating or editing a topic, select the Only members of selected Group radio button in the Post to to specify which groups will be able to view the question topic. 

If a question topic is limited to a specific group or groups, the Group icon will display next to the author’s name, above the question topic summary, to indicate that it is a group topic. Also, if a student is not a member of the selected Groups for a given question topic, the student will not see the item in the list of All Questions.

By default, instructor and teaching assistant roles will see all group topics, regardless of group membership. (However, the “View and group topic” permission may be enabled or disabled for specific site roles if desired.)