Ftp – Write permissions to multiple users on same directory in ProFTPD

According to the docs, you cannot set the SGID bit in the umask directive; it says: "Any arguments supplied must be an octal number, in the format 0xxx."

http://proftpd.org/docs/directives/linked/config_ref_Umask.html

As joschi alludes to, what you really want is "Umask 0002" - a umask is bitwise to the object, not an absolute CHMOD setting. You only need to specify it once (not "umask 0002 0002") as the first will apply to all.

You will have to give your proftpd virtual users ownership of their files and directories just like you would a real user. You can do this with chown. As the users aren't system users you will have to use the numeric UID:GID e.g. if username is UID 10001 and GID 1111 (from the mysql database) then

chown  10001:1111 /var/www/vhosts/sites/username
chown  -R 10001:1111 /var/www/vhosts/sites/username

should give username control over their own files. Ensure that www-data user has suitable read only access to the file & directories e.g. give world r-x to directories and r-- to files.

If you set the

DefaultRoot  ~

directive then the users will be confined within their own directory tree.