Other members of the discussion group will be able to comment on any of the posts. You’ll just need to login as any member and then go to:

Menu > Collaboration > Discussion

1. Select the group you want under My Group.
2. Click on the post under All discussions.
3. Now you add your comment and click submit to post it.

Once it’s submitted, you will see the comment(s) along with the user’s name under the comments section.