Default in Debian: ProFTPD only uses the /etc/passwd system user base.
To add "virtual" users :
In /etc/proftpd/proftpd.conf:
DefaultRoot ~
AuthUserFile /etc/proftpd/ftpd.passwd
RequireValidShell off
Then restart proftpd: /etc/init.d/proftpd restart
Create users file:
vi /etc/proftpd/ftpd.passwd
username:HASH:1011:1011:MyUserName:/home/ftp/directory:/bin/true
You can also generate the user lines with the ftpasswd command, but I think it's simpler to generate passwords with the command:
mkpasswd --hash=md5
Then paste the HASH in the passwd file.
Correct passwd file permissions:
chown proftpd /etc/proftpd/ftpd.passwd
chmod go-r /etc/proftpd/ftpd.passwd
And just test it to finish.