the server only seems to recognize the new OS as a guest, not a user
The OS is not a user, the user account you log on to the OS is a user. Create a user account on it, add that user to the file rights on the server. You can't set rights on a folder based on the fact that it's Windows 8 or 7 or anything like that.
Are the user accounts you use local on the server? Network accounts? On the computer? Try to map a drive using the "use other user" option, and authenticate as a user you already setup, that should work.