Google released a new feature that will automatically update Calendar events when changes are made in associated Google Groups.
As stated on the Google Apps Updates Blog:
...when Google Calendar invitations are sent to members of a Google Group, the attendee list will automatically update as people join or leave the group. When you join a Google Group, you will be added to all of its meetings. And if you leave a group, those meetings will be removed from your calendar. Note that groups stop dynamically changing when the total number of guests in the event crosses 200.