I am having the same problem. No issues with OX X 10.7.x or currently with iOS 5.x but upon upgrading to 10.8 and 10.8.1, iCal reports constant errors connecting to exchange server, including the " invitees not recognized" error and also "Exchange can't save the event to the exchange server" errors. Please address, Apple.
Here is the console output:
9/8/12 1:01:53.892 PM CalendarAgent[34024]: [com.apple.calendar.store.log.exchange.queue] [error = Error Domain=CalExchangeErrorDomain Code=-19 "“Breakfast with Michael Zimbalist” had invitees not recognized by the server." UserInfo=0x7fcb21ca7290 {NSLocalizedFailureReason=You entered an invitee that wasn’t recognized by the server., CalFailedOperationKey=<CalExchangeUpdateCalendarItemOperation: 0x7fcb25525940>, NSLocalizedDescription=“Breakfast with Michael Zimbalist” had invitees not recognized by the server.}]
and
9/8/12 1:01:54.109 PM CalendarAgent[34024]: [com.apple.calendar.store.log.exchange.queue] [error = Error Domain=CalExchangeErrorDomain Code=-4 "Calendar can’t save the event “Breakfast with Michael Zimbalist” to the Exchange server." UserInfo=0x7fcb233e9870 {NSLocalizedFailureReason=The account "Venrock" currently can’t be modified. To discard your changes and continue using the version of your calendars that’s on the server, click Revert to Server., CalFailedOperationKey=<CalExchangeUpdateCalendarItemOperation: 0x7fcb23298340>, NSLocalizedDescription=Calendar can’t save the event “Breakfast with Michael Zimbalist” to the Exchange server.}]