I'm in the planning stages of a new webapp, and I am leaning strongly toward a multitenancy model. The app has a file storage function, where the user can upload (and operate on) files.

I would like the ability of the user to share these files, however. How is this typically accomplished in a multi-tenant model?

The example would be something like google docs. Each user has their own files; they can edit and tag and build collections with these files. Then, they can share a doc or a collection with someone else for collaboration.

If every user has their own Database and tables, what strategy would one use to allow this kind of sharing while minimizing duplication of files and associated metadata?