Monitoring users connected to FTP
If you are using the server.app from Apple to turn on FTP sharing the log can be found in /var/log/ftp.log or if you prefer in the console.app (in /Applications/Utilities).

If you have turned on FTP manually using terminal (shell) commands you will need to configure logging in the FTP daemon. Info on that can be found by typing "man ftpd" in any terminal window.