1) One person needs to be the group manager and set up the group. I created mine on Google groups. You can set up a calendar and permanent reminders to help you keep on top of all submissions.
2) Group manager sends out email reminders each week as to whose turn it is, and who still needs to turn in critiques.
3) You can either invite people you personally know or send out invite on the SCBWI blueboards (or through your local writers' listserv). You might have certain criteria to join, or you might ask potential members about the number of years writing/if they've been published and/or agented before, etc. Accept/deny depending on whatever details you deem important. You might want to say members will be accepted on a trial basis for two months to make sure everyone is compatible. (No one wants a Debbie Downer who can only find negatives without being able to articulate that in a non-threatening way).
4) You might want writers of the same general experience level and intended age group, but it's not a requirement. Our group (Writing Buds) is mostly MG and YA, but there's an occasional PB in there by one of our members. *waves to Katie Sparks. We all critique whatever you want to submit, but YA writers might not be as familiar with PB or rhyme scheme, and PB writers not as familiar with theme or teen voice, for example.
5) Our group limit is 8 (2 critiques per week) and limit it to 10 pages/author. Other groups allow much more, but 10 works for our group. We track changes, write both positive comments and suggestions, and reply back to the whole group with the attachment in case others want to see what you've said about the others' work.
6) And that's it! We submit on Monday and ask that your critique is in by Sunday, so it's a clean slate
Hope this helps! Feel free to comment here or send me questions. xoxo, KYM