When Windows gives you the grayed out Guest username, you have 2 choices:
1. You need to create a stored network password on the windows computer.
To manage stored user names and passwords, follow these steps:
a. Open Control Panel
b. User Accounts
c. Stored User Names and Passwords
e. Enter the share path, username, and password
It will now auto use those credentials when connecting to that share path.
2. You can add the following to the share section of your smb.conf file
This method is not as secure as it will allow all the computers in your domain to access it without a password.