Use mount with the bind option
To have people chroot in there own privat space, and still have access to other shared space you can use the mount feature with the bind option.
This is what I used:
/ftp/support (this is the jail-root for the support desk)
/ftp/support/client1 (this is the jail-root for the client)
/ftp/support/client1/public (this wil be mounted with the bind option to point to /ftp/support/public)
The entry in my fstab looks like:
/ftp/support/public /ftp/support/client1/public none ro,bind
Unfortunately the ro (read-only) feature is not correctly implemented, so the user will have the file system rights.
We solved this by adding the user client1 to the group support, and we gave the group support read-only rights to the public directory.