What is the best way to have a private beta test?

We are getting close to releasing TagPro on Kongregate. We went the extra-mile to allow players to optionally merge their existing TagPro accounts with their Kongregate Accounts. So that playing the game in either place will result in the same account being played. This, along with many other features, needs to be tested well before going live. Luckily we have a great community around TagPro willing to test it. I currently have the Kongregate game pointed at test servers and want to allow the TagPro community to test it. What is the best way to open a private beta test? Even if was just a link, that would be fine. I basically want to expose the game to a certain group of people without the game showing up on Kongregate proper.