iCal as a collaborative office tool

We're trying to use iCal as a collaborative tool in our office. We have a senior exec who has two assistants that need to view and post appointments to his calendar. We have an Apple server and are using iMail and iCal as our office programs. We have not been successful in allowing all three people to access the senior exec's calendar and be able to enter, change, modify and view his calendar. We currently have it set up through a .mac account so everone can view it, but if anyone enters a calendar entry there it appears that nobody else can then modify it.

To make matters worse, each person has two computers they work from - a desktop and a laptop. This has required the to each have up to six calendars that they subscribe to just to work through this for the senior exec's calendar manegement.

Is there another way to allow us to manage the calendar where three people can get in to the same calendar, change it, modify it and view it (similar to what you can do with Outlook?) or is iCal simply not designed to be an enterprise level collaborative software program for this type of environment?

Thanks!