Hi Nick, first of all this is an awesome application!
It would be very nice (and probably easily achievable) to give the Admin an option to create events in which specified user groups can participate or public events in which anyone can participate up until a given number of participants. I haven't thought this all the way through, but this could be a rough "sketch".
An example, lets say we use this app for tennis court scheduling.
The administrator creates an event everyday from 05/02/2012 06 pm - 11 pm until 10/02/2012 06 pm - 11 pm, on courts 1,2 and 5 which leaves 3 and 4 open for reservation.
If a User Group is selected to participate, an email regarding the event is sent out. They should confirm or decline their participation.
If the event is public no emails are sent, only an announcement on the main page, where users can click and check the events description and details and click to participate or not do anything if they choose not to participate.
This event should of course have different colors on the schedule page.
I think this feature would be very good if implemented
Best regards Nick and all who contributed!