Content Rule to notify user when added to group or role

Has anyone heard of a content rule that works when someone is added to a role on group. I know there is one for notifying people already in a role or group. I'd like one to notify them when they are first added to a group

everything about content rules is geared around changes to "content" objects. It hasn't been generalised as a generic event mechanism for other aspects of plone like users being added, group changes, themes uploaded etc etc.

What ways are you allowing these group or role changes (Ie. you created a particular page or form in your site that does this, or are you trying to capture these changes regardless of how they're done)?