I would like contributors to discussion threads to be able to choose whether they want to be emailed when someone else adds something to the discussion - in just the same way as you can with this forum.
Is this possible? If so, how do I set it up? Using a space rule?
There is not an out-of-box way to do this, but we are looking to do the same type of thing for content. It would require a subscribe-type option on the discussion or the forum. I saw in the forge that Hans Verreydt has created a subscribe project. I haven't tried it yet so I don't know if it works with forums/disucssions.