[hadaso]

This is wrong! See below.

You cannot add another user to a "grandfathered" account, or make any change.
Until several years ago, a group account (family or business account) could have several users, each with the service level according to the user's needs.
Then it was changed so that all users must subscribe to the same service level, so now all users must use the service level that is needed by the user with the highest needs.
Existing accounts could keep their existing structure, but no change is possible without changing to the new scheme. If a single user needs a bit more space the all users have to get the same service level as the most heavy user.

I have a "grandfathered" family plan. One "enhanced" user (myself) and 3 "full" users. If I want to add storage to one of the "full" users I would need to change everyone to "standard" and it would cost about twice what I pay today. None of them needs "standard". "basic' is enough for them, but I need "standard" if I cannot keep "enhanced" because i use my own domain. So for the time being I just share a folder with my son so he can keep big emails there without going over his quota.
It's possible to go back 15 years and so what we did before there were family plans: get individual accounts and use forwarding rules to allow everyone to use addresses in the family's domain.