community.livejournal.com/dokuga_contest
It's open to everybody as long as you're an LJ member. Membership is free and it's easy to join, though I'll warn you it's not the most user friendly until you figure it out. It took me a little while to figure out how to post.
At the top of hte page you'll see a link that says 'user info' or 'profile' (I can't remember which) if you click on that it will take you to the profile page and all of hte rules are spelled out there.
On the sidebars of the community it tells you the challenges. There is a weekly word perfection challenge so you interpret the theme however you want and just make sure you hit the word count exactly.
There's also a bi-weekly one shot challenge which is 500+ words.
Forthright has also created several ongoing challenges that can be completed at any point in time. She offers participation banners to those who complete them. There's the seasons challenge, spectrum challenge, christmas/holiday challenge and Child's Play. You'll see links for them on the side bar as well.
If you have any questions about the community, you can send a PM to forthright, or send a PM to me as I'm familiar with it, its set up, rules, etc.