unable to post despite all permissions set to be able to

I went to the “What’s Your Favorite WordPress Theme?” at the Automattic Lounge in San Francisco yesterday. I was introduced to P2, and I really like it, but I am totally stuck.

I have everything set as I need to, but unfortunately, I am assuming that I am not understanding how to set the site up so users must sign up to use the site.

Do I need to make it a MU site? is there a particular plugin that allows users to sign up and post?

Something seems off, I have all the permissions set so that any person can post, however: A. they can’t register for the blog, and B. they have no ability to post.

the site name is http://YogaRetweet.com –

Please let me know,

b