The Groups addon allows you to share projects (folders) between users with precise controls.

Share with Roles

This added functionality allows administrators to assign all users of a certain role read, write, or delete permissions to a folder.  (Controlled from “Documents > Folders”)

Add a Specific Person to This Folder

This area gives administrators the ability to assign read, write, or delete permissions to individual users.  (Controlled from “Documents > Folders”)

BuddyPress Groups

When BuddyPress is enabled, users can create groups. When activated, this plugin will detect all groups available, and will let the administrators decide whether or not they want to share a project with a group.