ftpuser can only upload files in /home/ftpuser to have users only add files at /home/somedomain.com/public_html
Add new user say you have a client at www.domain.com
sudo adduser newcustomer
sudo usermod -d /home/mydomain.com newcustomer
sudo usermod -g ftpgroup newcustomer
sudo apt install acl
setfacl -R -m u:newcustomer:rwx /home/mydomain.com
## sudo chown -R newcustomer:newcustomer /home/mydomain.com
EDIT: Allow users to see subdomains if they choose not to point them to a subdirectory inside public_html
EDIT: changing permissions should allow the owner (root/user that created website) to have same access.