Clearspace does not have the functionality to do this. If a user wants to get email notifications on updates to a discussion, they need to go to the discussion and click the "Receive Email notifications" option in the "Actions" box.
Note that if a user creates a discussion or makes a reply in the discussion, they will be set up to get email notifications unless they have changed their profile preference settings to not be notified. However, if a user is not a participant in the discussion and has not chosen to be notified for updates on that particular discussion, there is not a way to set Clearspace to email them.
Has this functionality been added yet ?