If you're a writer, being a member of a critique group is indispensable. Critique groups range from formal to casual and it's best to join or create one that suits your personality. In fact, to maintain the tone and success of a critique group, it's not a bad idea to have a few rules. What follows are a few suggestions.
You may want to have a confidentiality agreement for all members. Note: this may seem a bit paranoid, but some people swear by it. It is merely a measure to discourage plagiarism and keep people honest. Take it or leave it.
It's not a bad idea to ensure that members of your group are at similar levels of skill and commitment. One way of doing this is by having membership requirements, whether that's obligatory membership to a recognized writer's association or the completion of writing classes beyond high school. You may want to limit the number of members in the group. Other things that may fall into this category are: minimum attendance, output, and feedback at meetings.
You may want to have manuscript requirements. Examples include genre restriction, numbering of pages, minimum or maximum number of pages submitted, requiring manuscripts to have been proofread for basic spelling and grammar errors before submission. Some of these suggestions may sound a bit uptight but are all ways to get writers accustomed to doing what must be done for submitting a manuscript to an agent or editor anyway. It is just one of the ways critique groups can shape you into a better writer.
Other rules that often go into a critique group involve the critique process itself. Consider submitting by email in advance to give others a chance to review material before giving comments in person. Do you want only one person to present material or everyone? I have found that 3-4 people can reasonably present new material (read ahead of time by the others) and get feedback within a 2 hour meeting if there isn't too much chit-chat or snacking. Add half an hour to an hour if you meet at a pub. Do you want to read submissions out loud? This may be helpful for difficult passages. Make sure members write comments on the manuscripts they've printed out and critiqued. Don't interrupt the person giving comments. Wait your turn, and, if you're the author, be careful not to be argumentative or too defensive of your baby. You'll hear this many times, but if you hear a specific criticism more than twice, PAY ATTENTION!
Above all, as a member of a critique group, remember that criticism should be given with the goal of helping the writer improve the telling of the story. Do so with a generous spirit. As an author, remember not to take criticism personally. You should be able to take harsh criticism as well. If you are easily discouraged and want to quit writing, you probably should. This path is not for the faint of heart.