FTP is not secure and should be avoided. Secure FTP comes in two flavors, SFTP & FTPS. What you are looking for is better achieved by permitting users SSH access. SME has support for SFTP via their implementation of SSH. Users can log into their own home directory tree and in addition may traverse into any ibay they have group rights to.
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However, bear in mind SME is limited in granularity of SSH/SFTP security options. If you wish a more granular ibay security set up where one user may upload only, one user may download only, one user has full access rights, this is beyond SME defaults.
Darrell