When you create project files outside of Code Co-op, save these files within the project directory tree. You will then be able to view these files from the Files tab.
Select the files and click the "Add Selected Files" button
from the toolbar. Once you check-in these files, they will officially be part of the project.
In this screen you can see we're adding two files to our project.
Select the Create File button
. You will be prompted for file name and type.