Hi resman, Yes you can apply to the challenge at any time, I started a few weeks ago. Make a post on your site about joining the challenge and post on the new challenger thread.
Once you have hit 6 months and have dropped below 200,000 Alexa rank, then you need to let the forum know and send a message to Sam from Financial Samurai for inclusion in the next class. They add new members twice a year.
If you are accepted, then you will be invited to write a member post. Anyone more familiar with the challenge details please feel free to correct if I am mistaken on any of these points, this is just the information that I have gleaned from this forum.
I just wrote what is in my opinion (of course I am biased ;) a really good post on 7 Options For a Tax Free Early Retirement. Also check out my older posts here.
Do you have a twitter account for your blog?