I'm using proftp and I have a file called '/etc/proftpd.conf' with the configuration of the server.
I have a line which says
which makes the home directory of every user to be their root directory when they are connected through ftp.
You can change thi line and instead of '~' write the directory you want.
I think you have a similar file to config the ftp server, just go to the documentation and look for it.
... I hope I'm answering your question