The first step is to install a FTP server. You can find how to do it here: https://secure.servergrove.com/clients/knowledgebase.php?action=displayarticle&id=153
Once the server is installed, you have to configure the users that will connect with FTP.
To add a user with permissions for a specific domain, you have to set the vhost directory as the user's homedir.
useradd -m -d /var/www/vhosts/example.com/httpdocs USERNAME
or if you want to disable shell access for the user
useradd -m -d /var/www/vhosts/example.com/httpdocs -s /sbin/nologin USERNAME
Now the user is created and is able to connect to the server with any FTP client.
IMPORTANT: Shell access will allow the user access to all readable files on your server.