I agree with Dave. Actions such as adding and removing notifications when a user is added or removed from a group is not a dotLRN specific need. What is .LRN specific is probably exactly what is added or removed.
Here is a non-.LRN use case, and I just consider this one of many possible use cases. On Aristoi I'm using subsites with bugtracker mounted for various clients. I'd like to set up bugtracker so that when a user was added to the subsite he was automatically subscribed to bugtracker and definitely I want the user to be automatically unsubscribed if he is dropped from the subsite.
Jeff, does COP do any sort of similar operations when a user is added/removed from a group?