After the latest update to Outlook 2011 (14.3.2), Mac users are unable to send emails to contact groups. They receive this error:
EmailAddress or ItemId must be included in the request.
I haven’t seen a fix or workaround from Microsoft yet.
Update 3/25/2013: A recent update to Outlook 2011 added the ability to expand a contact group into its constituent members after you’ve added it to the To field. (You’ve always been able to do that in the Windows version.) This is a decent workaround until the root problem is fixed.