I can't send email to groups now that I have added iCloud to Outlook; groups aren't found anymore
You might be facing the synchronization problem:

Microsoft chose not to support CalDav and CardDav in Outlook for Mac, unfortunately these are the services required by iCloud. 

Although MS did add these to Outlook for Windows they did not add them to Outlook for Mac.

In short you can't subscribe (not sync) to iCloud from Outlook for Mac (although you can if you use Outlook for Windows)

Use Outlook, or use iCloud, not both.