Probably a number of you know about this information but it is newly discovered for me today.
I could not for the life of me understand why I could not choose a preferred email address for the people in my Address Book Groups. It did not make sense.

These two articles describes the problem, why it happened and has noted that Apple is aware and will be working on it.
At least I now feel enlightened and understand the issue.

This only applies if you are using iCloud to manage and sync your Address Book.
I hope Apple fixes this very soon.

Address Book distribution list editor disabled by iCloud | MacFixIt - CNET Reviews

How to email groups with Mail - Apple, Mac, iPhone, iPad, and iPod Reviews, Help, Tips, and News | Macworld Australia

Sorry if this information is a repeat for you, but I still see a number of posts regarding this question.