Update email notification subscribers

I'm not sure if this is already possible.. if it is then the interface to accomplish it isn't clear.

When a new message is being added to a project it allows you to set who you want to have notifications sent to (setting the subscription list). Once you've added the message, you can not update that notification/subscriber list.

I recognize that members can add themselves to the subscription list by visiting the message, however without the first notification they often are unaware a new message has been added!