An update when a change has been made in a workspace is a great idea -- though you wouldn't want to have it for every time the document is saved.
I'd vote for the earlier of when the user who made the change signs out (sending an alert to all who have shared privileges) or at some predetermined time (which ideally could be set for each user).
And the option for those notifications to be sent as SMS messages would help it to be removed from the clutter of my inbox, and assure I'd see it when it happens.