I'd like to create on my N900 a new user that's very restricted:
having read-only rights to the /media/mmc1/DCIM/mp3 directory, & being unable to see, go to, or do anything with the rest of the file system.

The purpose is to create a guest login to allow sftp: connection via Rhythmbox on another computer to play music from the N900.

I've seen pages on the Linux "adduser" command, but haven't been able to figure out how to give the user such a narrow scope of rights: read-only in only one directory (and its subdirectories), & nothing else.