You can still join! Even though the first round has already started, you can still take a shot at becoming America's Next Author by signing up and entering a short story (2500-5000 words). There will be eight nomination rounds in total, so if you sign up this week you'll be able to participate in seven, which is still plenty of time to get a lot of votes and support.
Joining this week might even give you an advantage because you can look at what the current participants have submitted. Don't be afraid to join -- there are almost 200 entries so far and we expect the number to go up each week. The authors that join in the first fews weeks have the best chance of winning!