to change the default login directory for vsftpd, change the ftp user home directory in /etc/passwd:
ftp:x:116:116:vsftpd daemon:/var/vsftpd:/bin/false

Source

useradd example – Add a new user to primary group

To add a user tony to group developers use the following command:
# useradd -g developers tony
# id tony
Sample outputs:

uid=1123(tony) gid=1124(developers) groups=1124(developers)

Please note that small g (-g) option add user to initial login group (primary group). The group name must exist. A group number must refer to an already existing group.

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