To add a user for ftp purpose only so they can log into their home dir and do nothing but upload and download.

use:

useradd -m -k /dev/null -s /sbin/nologin {username}


for web use - if your hosting a www site for them and only want to give them ftp access

useradd -d /var/www/html/{username} -m -k /dev/null -s /sbin/nologin {username}