Check out this thread, which has some resources.
The quick answer is that the user has to have access to the project to see the workbook at all. You'll need to configure permissions within that project to grant permissions to all other users and deny workbook permissions to your one special user.
Try this page for setting up groups for permissions:
Actually, you are not locking the permission at the project level so, you can assign the permissions to individual workbooks.
Please note, you are allowing Publishers/Explorer(can publish)/Creator to modify the permission for their dashboards within your project.
The best approach would be to create another project, add existing group and additional user, lock permission to the project level.