A few of my users are other server admins with Plex Pass and their own server. They have families and their own managed users.
When I add such a user, Id really prefer it if I could add just that one user and have the his/her managed users automatically get the same access. So that the user can manage his own family set-up without my interference. Is this not possible? Do I have to add every user manually?
No, this is not possible.
The other admin has no influence over the rules on your server, regarding age ratings and sharing restrictions. Important for parents.
The only way would be to create full plex.tv account for the other family members (which is probably what you are currently doing).