Adding a delegate to Apple iCal

Hi

I'm in the process of trialling Office 365. All users will be on Macs with version from 10.6 - 10.8. 

Each user will be sharing their calendar with all users. The problem I'm having is that I'm unable to add a shared calendar under the delegate option in iCal. A previous posted suggested that you add it manually by clicking to '+' and typing the name of the user - this does not work.

So is there a way to add a delegate in iCal for Office 365 shared calendars?

Thank you