Yes, Katie is exactly right.
There are two roles – Writer and Reviewer. The Writer is totally in charge – s/he creates the Group, and can invite one or more Reviewers into it. I expect that many Groups will consist of only two people, but more is possible. And of course, you can create a Group, work on the feedback, and then create another Group for the same project a little further down the line, for more feedback, from other Reviewers.
It’s much more flexible. There is no bar to entry, as there was in the original Houses system (which pretty much mirrored the publishing industry as it was then). And the overwhelming emphasis is on developing powerful, effective writing. Whichever route to market you choose, traditional or self-pub, it’s the power of the words that really counts with today’s reader.
So... more of a laboratory, less of a beauty contest.