Bugs can be fun to find... except the kind that are in the form of unhandled exceptions.
How are you going to choose the top 100 people... i'm not sure how many people are registered here exactly..
As he said, there would be a sign-up on the forums, and they will give preference to those who have shown the most interest. While there are a lot of registered users on this forum, there aren't that many active accounts. In fact, there aren't nearly a hundred regular posters around here, so if you're a genuine fan who is giving feedback right now, your chances of getting in (if you want to) are fairly good.
I can tell you that this will be a very unique opportunity, but you can't go into it thinking "free game". This won't be a finished commercial product, and if you treat it like that you're going to get infuriated by all the bugs and, invariably, crashes. Instead, you have to view it as a chance to be involved in the design process of the game itself. That's something you can't buy on the shelves of a store.