I've created a new user for FTP.
useradd -g www-data -d /srv/www/vhosts/project/ black
I made a mistake, I actually need the user to be in the group
How can I change the group?
You can use:
sudo groupadd www # Add a new group, if not exists usermod -a -G www black # Add the existing user black to www usermod -g www black # Change the primary group of black to www
To confirm that it has been added:
From removing user from secondary group, in this case
gpasswd -d black www-data