So there's a way to create a new user change ownership of the site a custom group etc etc
unsupported and I'm not doing that.
Since no-one answered this is what I did that solves the problem.
I want another account to be able to upload pictures into the site without complete access so what I did is just create ssh keys so AAA can rsync (pull) from BBB the files that it uploads with a cron that runs every 10 minutes.
i.e. AAA cron rsync -a BBB@0:/home/BBB/ftpfiles /home/AAA/ftpfiles