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.
  4. Click on the post under All discussion.

Then it shows the post and will get a box that will help us to submit our comments. Refer the figure shown below.

After submitting our comments it will shown under Comments.

For more information, view this :