There are two options to participate in an existing discussion. Choose either a recent discussion from the navigation of any space or group page and choose from a list of all discussions by selecting the Discussions option. You can also filter the discussions by date created, newest to oldest, latest activity, or in alphabetical order.
Once you select and read the Discussion, you can click on the Comment link at the top of the discussion and it will drop to the bottom of the discussion post opening the comment window for you to begin typing your reply.
Once you have typed in your comment, click Add Comment to post.
Alternatively, you can click the Add a comment/reply button located at the bottom of the post to open the Add a Comment/Reply window.
Type your comment in the content body and click on the Add Comment/Reply button to post.
You can also Use @mention to tag people, places, and other content within your comment.
For additional resources for comments, please read: