You can add invitees to your events in iCal. This will send those people whom you invite an email for the event. Read about this feature in the help menu of iCal, also here: http://docs.info.apple.com/article.html?path=iCal/4.0/en/9863.html
The other option is if you and the Outlook users both have access to the same Calendar server (Exchange) you can add your Exchange account to iCal: http://support.apple.com/kb/HT2513
Hope that helps.
Thanks for the info Chris.
That process edits a script embedded in the iCal application. I would use caution editing the application. That is not an Apple article as you can tell from the wed address you provided and regardless of it's source, Apple would not suggest modifying their applications as it may have unexpected results.
If you are going to attempt that process my advice would be to archive the iCal application (highlight it in the Applications folder and go to File > Compress). Should iCal stop functioning after you take those steps or have other issues you can delete the current copy and then open the compressed copy and apply the last combo update for your OS found: http://support.apple.com/downloads/.
Best of luck to you.
I am having the same issue when sending iCal event invitations to Outlook users (both are on macs). They just don't receive any email invite to reply to, so remain unaware of the event.
I cannot see a way of checking whether they have even been sent, as non-Outlook users receive the invite emails fine.
Is there a log somewhere that lists the invites sent out perhaps?
I can't believe it's impossible to invite an Outlook user to an event via iCal on a mac, any suggestions would be gratefully received.
I'm using OSX 10.7.5. iCal is linked to an iCloud account that used to be a MobileMe account.
I tried following the instructions in: http://www.ehow.com/how_5887667_send-ical-invites-outlook.html, but couldn't get the code to "compile" without errors, so reverted to the original.