Generally, the more narrow the scope of the group, the better. If you’re writing poetry, then your group should be limited to poetry. People who write in other genres will probably not be able to give you quality feedback. It is best if you can find a group that writes only in your specific genre but you’re probably going to have to choose between three general categories: fiction, non-fiction, and poetry.
If the group is too large then everyone will not have regular participation. If the group is too small, then an absence by one person will adversely impact the quality of the meeting. Also, different members will have different strengths and will notice different things. One of our members saw point-of-view issues while another member saw continuity problems. I recommend no fewer than six people and no more than twelve.
This is partly a matter of where you are as a writer and what you want from a critique group. If you are a developing writer and want regular feedback on a work-in-progress, then the group should meet every week. Once a month will not give you enough feedback to develop your craft. Also, meeting less than every week will easily turn into commitment problems.
Group members should attend every meeting and participate. If it is your turn to read, then bring material on which you have made a genuine effort. If it is your turn to critique, then give quality feedback to the writer. Of course there will be absences but members should know about these in advance. If a member cancels at the last minute because he has to pick up his wife at the airport, or if someone forgets about the group and schedules a work meeting at the same time, then their position in the group could better be filled by someone else. An occasional absence, with advance notice, is expected. Even longer absences can be accommodated. Our group was blessed with the arrival of two babies!
This is the most difficult factor to control when forming a group. I was fortunate to have all quality writers, including published writers, in my group. A weak writer, who gives unhelpful feedback, is not productive and will not help you grow in your craft. You’re probably not going to have Pat Conroy or Amy Tan in your group, but members who are quality writers and make a genuine effort to improve their own craft, will help you improve yours.