But what about ad-hoc group planning?
"Hey, anybody wanna get lunch this week?" "Sure!" "When works for you all?" "uhhh..."
So, how about something that lets a person propose an event to a group. They all get emails about it, can express disinterest (which means they won't be mailed about it anymore) or say when would work for them. Depending on the event, they could invite other people, and they could invite others, etc, and the system can kinda manage itself. You could see when people would be able to show up, both individually and en-mass, so you can see what the best options are. Again, depending on the event, either the originator, or the first-tier invitees, or a vote, or the system itself (based on majority availability) would decide on the schedule and the system would announce it.