From the site:
A burst is a short piece of writing, 1,250 words or less (written and optionally spoken), typically written in 15-30 minutes (but it’s really up to you). You can start with a random sentence from a book or from scratch, the only rule is that it’s written without stopping to go back and correct yourself. Once you’re done, if you feel like sharing it, then you can post it to Burrst.
Writers must be invited. If you want an invite to write, you can request an invite and/or register as a reader and let other members know in your bio that you want to be invited. Adding a link to your smashwords.com profile or a website of your work can’t hurt your chances either.