SOLVED!! you must go to each FAMILY members icloud account (or have them go there) select iCLOUD Settings > Custom Email Domain > Manage --- see the NEW domain, click it, and get the + on thier account! Add their new email address!!
I created my "family" sharable domain, and was only able to get it started with NO email addresses since mine wasn't an existing domain email for my new domain.
Once I got through Apple's setup steps and my new domain was verified, I was THEN able to add my new account under my new managed domain.
NEXT, I was able to get my wife's new email domain address added FROM HER family share account. From HER account, in iCLOUD Settings > Custom Email Domain > Manage --- I see the NEW domain, click it, and I get the + on HER account!
Yea!!!