Recommend Calendar Hosting Service

There are diverse salespeople who sell my wife's artistic services and those of other photographers like her. They are looking to replace their currently self-hosted calendaring system with a hosted service. Can anyone here recommend one or a place for me to look? Here are some of the features I imagine they are looking for:

1) Individual calendars by photographer.

2) Easy color coding of items on the photographer calendar. They color code based on which salesperson found the job and if the job has a hold or a confirmed status.

3) Integration with iCal on the Mac. At a minimum, iCal users should be able to subscribe to calendars on the service.

4) Various levels of security. All salespeople should be able to update all photographers' calendars. Individual photographers should be able to see only their calendars and may (or may not) have permission to change them (this is determined on an individual photographer basis)).

5) Reliability. This is at the heart of their operation.