I need a simple CMS software (or maybe it's called something else). There are many users (adding new users has to be simple as it will be done manually, no "sign up" link) and those users have to access (just download, no uploading or modification) some files. A file can be shown just to one user or a group of users (a user can belong to multiple groups). Upload a file and tag which users/groups can access it.

This can probably be done with FTP, but HTTP would be better.

Any software (preferable open source) that can do this without much modification? I can modify PHP code a bit, but I am not a programmer.