Recommend using adduser (instead of useradd) for adding new users. It’s more user friendly.
adduser xxx
Modify user’s home directory
usermod -d /var/www/directory xxxusername
Modify user’s home directory while copying existing home files
usermod -m -d /newhome/username username
Give appropriate permission to new directory
sudo chown -R newuser:www-data /var/www/new/directory/
Add userX to group sftp
sudo usermod -G sftp userX
If you wish to grant sudo privileges to new user
sudo usermod -aG sudo newusername