You can setup the Admin and Guess passwords through the Slingplayer Desktop's Setup Assistant.
However, access for Web Slingplayer (http://watch.slingbox.com) is govern by the Sling Accounts (https://slingaccounts.slingmedia.com). You'll just need to provide them with the Slingbox ID and guest password. Then have them to create thier own Sling Account and register your Slingbox with the information that you gave them. They will be able to use the website with the guess password.