We've just released a few improvements to our group events module, based on some of your requests:
1) Max Attendees: You can now specify how many people can attend an event, including yourself. Once the limit is reached, group members will no longer be able to RSVP 'Yes' and the event will be marked as 'full'. Obviously, if someone cancels, that spot is automatically back in the pool.
2) Close an Event: Let's say that, for whatever reason, you no longer want any new group member to RSVP your event. Just edit it and un-check "Event is open". That will prevent all future RSVP. You can use this feature for example if you want your group members to RSVP before a given deadline.
3) Past Events: Past events are now automatically closed, which means that RSVPs cannot be modified after the event is started.
Let us know what you think and don't hesitate to suggest additional functionality.