It sounds like you upgraded to Premium for Family and one of your family members can't log in. Keep in mind that in a Family plan, all family members use their own account to log in, so having changed the subscription to Premium for Family shouldn't affect the login details of your family members. Make sure that the member uses the correct log in details (if they don't have an account, they can first create one here).
After that, make sure you've invited them to the Family plan. They should accept the invitation while logged in on their own account. Check out the steps here.