So, this post is from Nathaniel Kressen, the leader of my writing group at Word Bookstore in Greenpoint. Wanted to pass along, because we think about a lot of similar things 
- Frequency. Having a deadline compels a writer to produce more material than they might otherwise create if left to their own devices. Too much or too little time in between meetings can reduce the positive impact. In my experience, one week is too short of a turn-around time for a writer to absorb notes or create a body of new material, whereas one month can be too long and undercuts the urgency of the workshop, allowing the writer to procrastinate. Test out whether meeting every two weeks works for you and adjust as you see fit.
- Group size. The benefit of having a small group is that everyone has the opportunity to participate in the discussion, but limiting the number of writers who can join will only prevent you from receiving fresh perspectives on your work. The good news is that the number of participants will fluctuate meeting by meeting early on, so there should be plenty of opportunities to experience both and decide what works best for the evolution of the group.
- Structure. In some groups writers read their material aloud, but I’ve found that e-mailing work in the days prior proves more productive. First of all, it gives readers time to think about the material before offering feedback, and second, it presents work in the same way it’ll be seen by editors and agents.
- Feedback. Some groups elect to have writers sit silently while receiving feedback, but this may limit the constructive dialogue that can happen. The trick is for the writer not to become defensive, and for the readers to stay focused on what the writer is seeking to accomplish rather than what they would do differently themselves. If the dialogue remains honest, open, and patient, it will prove useful to the writer’s process.