Users "following" a discussion can view posts by other users against that topic. Once a discussion is followed, the user can add posts, attachments by even replying to the emails.

To follow a discussion:

Create or Edit and existing discussion topic.
On the New/Edit pop-up, click on the Followers drop-down to select users to add as followers.
Click on Save.

You can also click on Follow this item from the actions drop-down menu of a discussion.

To unfollow a discussion:

Create or Edit and existing discussion topic.
On the New/Edit pop-up, click on the Followers drop-down to remove the users to unfollow as followers.
Click on Save.

You can also click on Unfollow this item from the actions drop-down menu of a discussion.

On the actions drop-down of the discussion and clicking on View Details, shows you the list of users subscribed to the discussion.

Browser not supported