In the next few weeks, Google Inc. will roll out a change to make it easier for you to add or remove Google Groups members from Group Calendar invites.
Essentially, if you send a Google Calendar invite to members of your Google Group, the attendee list will automatically update as you add or remove people from the group. Also, if you join a Google Group, you will be invited to all of the Group’s meetings, and when you leave the group, all meetings will be removed from your calendar.
This change applies only to new calendar events created after the rollout. To apply it to existing calendar events, simply re-invite the group. For more information, see Calendar events that update when Google Groups change.