Not ideal, but you can use calibre's ability to manage multiple libraries to solve your problem. The assumption here is that ebooks are small in size (true) and disk space is cheap (was true, currently isn't true, will be true again).
From your "master" calibre library (the library you've already created):
- For each user you want to have their own library, create a new library in Calibre and name it appropriately. Place these new libraries on shares that other machines can access. You can ACL them so that only you and the intended user can access the shares.
- From the master calibre library, copy the intended books into the other libraries as appropriate.
- From each user's calibre installation, point it to that user's library location
Each library will have its own copy of books, and you'll have to manually copy any new or updated books from your master into the other libraries, but this will allow you to manage a "centralized" library while maintaining file permissions and not worrying about other users accessing the libraries at the same time (coordinate when you copy books vs. when they're using the libraries so that there's no overlap).
As I said, it's not ideal and it "wastes" disk space, but it's probably the best option for your scenario until calibre gets true multi-user library access (IIRC there's a work item for that, but no target date besides "When it's ready").