One of our users just reported this and, initially, I thought it was user error, but I'm seeing the same behavior myself in other Groups. Basically, when you create an event on a Group Calendar, there is a checkbox labeled "Send a meeting invite to group members". If you uncheck this and create the event, only those "following" the Group get an invite. If you uncheck it and then add/invite a specific person, that box rechecks and is disabled. So, it does not seem possible to send an invite to specific people without also inviting the whole Group. See http://www.youtube.com/watch?v=mNENw__PcPQ for a demonstration of this behavior.
There are manysituations where someone might want to add an event to a Group calendar and invite only specific members of the Group, or even someone outside of the Group, so I'm hoping this is not "by design" and is something that MS can fix.