A very simple member ship system is required.
Once the user is logged in,he/she has a section to to input a list of email address along with a password. That user also has a section to uploads a certain file.
Need a Rest Client
Only those email addresses which have there details given can have access to that upload area specially that uploaded file.
for e.g if one email address was registered by admin320, the user from the client side would get authenticated and get the file uploaded by admin320.
for e.g admin320 is one of the users registered at the site. And he has put in 5 emails and passwords in the items [url removed, login to view] user from the client side would get authenticated and get the file uploaded by admin320.