They invite 72 teams and many are well known and ranked. This does not mean they don't bring in other teams however.
My son's team finished fourth a few years ago and it was put together more or less for this, and a few other high profile events. The coach went to great lengths to show who he had on the roster and show that the team had many of the top kids from the reqion we were coming from. He also had some references from pro scouts. He was organized and presented a good case.
BTW we spanked a number of the "academy" teams on the way to the final 4 games. We were good however and probably 1/2 of that team will be playing D1 baseball somewhere next year.
So if you can bring a solid team and back it up you can get in.