I did not have an invite, but I just downloaded the google voice app from the market, went through all the setup and it automagically created a google voice account. Then, I went online to log in and at first it wouldn't let me, but the second time it did. From there, I was able to create a new google voice number and everything. I don't know if it was a fluke, but I thought it might be worth mentioning. Let me know if it works for you too.