As far as which folders or files they are using, that can be set up by modifying what the SFTP subsystem logs if you are using OpenSSH server. Set a Match block for the group of users and force them into SFTP mode with modified logging:
Code:
Match Group watched
ForceCommand internal-sftp -f LOCAL0 -l INFO
Banner /etc/ssh/watched.banner.txt
Then configure your system log daemon to sort messages with log facility of LOCAL0 into their own file. You'll really want to do that because the logging at that level is quite noisy.
However, if you don't want the users roaming around the system when logged in, you can chroot them into a specific directory or hierarchy of directories.