For the creator's program, you can add your own Xbox Live account, or any other Xbox Live account, to your sandbox. Thus you and any of your colleagues and friends can help in the development and testing of the game.
Once you have enabled Xbox Live for your game, scroll down on the Xbox Live page to the Test section. Look for the link to "Authorize Xbox Live accounts". Follow the instructions there to add the MSA ID.
The other instructions you'll see in MSDN (to create test accounts) is for ID@Xbox or our managed partners, which often have to create dozens or hundreds of test accounts at a go. These are pseudo-accounts for large scale testing scenarios.