Today user has no possibility to make settings how he want to be informed about todo. If the user is a manager and want to see what the team are doing he need to see a daily e-mail with all todo done/changed etc. The best if this todo will be incorporated into the daily features email (great Aha feature and your work here!).
Today when there is five persons assigned to a ToDo, user who reported the task will get several emails e.q. one of the user finished the Todo, somebody make a comment, etc. User working on this task need it. Managers just need to see what is going on on daily basis.