To post a reply:

Go to the discussion by running a project-specific discussion report or from a report.
Click on the Discussion or from the actions drop-down menu of the discussion click on Reply.
On the discussion's page, click on Reply button and enter your comments.
Click Browse to upload files, if any.
Click Save. Your comments are posted against that discussion topic.

Participating via mail

Once a topic is created, the follower gets an email notification. The follower can reply to the email to add posts to the discussion. Attachments can also be added when replying to the email. Once the follower replies to the discussion, other followed users will also receive an email with the post/attachment.

The email will show the entire thread of the discussion (upto 10 posts).

Browser not supported