Find your group and go to that page. On the left hand side you will see the “Forum” option. Click this. There will be an option for Topics. Click on the topic you wish to comment about. Scroll down until you see were it says “Reply To: Topic.” Type your response in that box. There will be a check box that says “notify me of follow-up replies via email.” Click this if you wish to get an email notification about replies to your post. If not, leave it unmarked. When you have finished your reply hit the “submit” button at the bottom right of the reply box. You will see your post pop up under the most recent one.