If you see an interesting post and reply at the bottom only the original poster will be notified of that reply via email as far as I can make out. However, if you want to add a comment to another person in the thread and you want to be sure they will see it, then leave a reply under their name and then they will be informed of that reply via email.
As we all have fibro we can't be expected to trace all the threads where we've left comments! If you definitely need replies it's best to create the thread yourself.
Hope that helps people. x