It sounds like you want to add an existing user to another group. Use usermodusermod to do that - usermod -a -G ubuntu www-data The
usermod -a -G ubuntu www-data The change will be in the /etc/group/etc/group file though. You can check it with id www-dataid www-data and groups ubuntugroups Ubuntu commands. Also make sure to, use the upper case G-case G, or you will replace the existing primary group.