In Mail, under Preferences you can elect to hide group email addresses when sending to that group. However, I do not want to hide ALL my group addresses, only some of my groups. Appears that I must either do all or none. Is that right?
You seem to be misunderstanding this feature. It allows you to hide the addresses that are displayed to you when you compose an email, but if you put them in the CC field, every recipient will be able to see them, which is why you should
never do that; always put them in the BCC field.
Mail does not allow you to hide Groups, only the display of addresses to you when you send an email.
Not sure that's true. When I have it clicked to hide addresses when sending ot a group, people that receive my email tell me it says Undisclosed Recipients in the email they receive. And if they try to hit Reply All, it sends only sends to me, not my group.