1.  First login as a student or any other user of the group. 
  2. At the top of your dashboard, click the module access icon> Collaboration>Discussion
  3. Select particular group under My Group.




 Then you will displays a window as shown in the figure below. Update contents and click on post.




For more clarification, please watch this.

IobU9f-dhpeDdeaFnVIa_uywIyYuhBsezg.gif