Last time i got this issue:
I had a friend asked me for some space, so i made up an Account on Virtualmin, got him some space, some ftp-users etc. Just after he asked me, when i upload some files onto my Server (using ftp) can i create new ftp-Users and give them access to specific folders/files i uploaded?
It would be nice to have such a possibility provided in Virtualmin (free and pro). i think it can be made using bind mount's and group/user access restrictions.