I'm on another website of this ilk where the focus is on small workgroups. There are no big groups. It's a very intimate experience, and I like it a lot. However, my problem with it, is that it doesn't allow me enough throughput. I am in the midst of a novel rewrite and I can easily post a Chapter every four of five days. I will never find a group that can deal with that kind of volume and provide effective reviews. I have solved this by being a member of several groups, and I use this website for my less refined work.
With that as background, what I have seen in Groups is they get difficult to manage over a certain size if you really want a disciplined approach as you are proposing. You also need VERY committed people, or you end up with a handful doing all the heavy lifting both in terms of submission and reviews. Also, without some kind of boundaries on subject matter, if you leave it wide open, you'll get such a wide variety of work that I think some people will get disinterested. For example, your historical fiction buff might not be too interested in the writer doing LGBT YA Vampire Fantasy stories.
In your prior case, you had people meeting face to face, and that puts pressure on people to fulfill their commitments. When you have people spread all over the world, people get less committed and tend to come in and out.
These are kind of random thoughts aimed at making you consider other parameters and think about how you can sustain commitment from people. In my experience, the best Groups I have been in are ones where the goals between the people are more aligned. The ones that tend to fail are where the goals are disparate.
