Click into a project, (can be proposed, accepted or active) click on the files tab and click add a folder. Type in the desired folder name and click add.
You can share this folder and all its future contents with the project's client by clicking share with client. You can unshare at anytime and even delegate file-specific access with your client.
If you'd like to create a folder for all of your projects, including future projects, contact EduSourced support and we will add this as a default folder for you.
Note that you cannot currently rename a file folder.