You may upload files to the Documents tab. Click the Upload button and choose a file from the pop-up. In tree view, you may right click on a folder and choose Upload to add a file directly to that folder.
The user will need view and edit permissions in documents to create, edit and upload to the documents section. Depending on the settings of a user's project permissions, they may be able to view all documents and folders, or just a subset of them. Entities specifically shared to a member will be viewable to them, and files marked as public will be viewable to all project members with view access to documents.