In Pen on Fire there's a chapter on critique groups that's pretty lengthy and informative, but I just received an email regarding tips for running critique groups so here's a few things to get you started:
* Meet the same place, same time, every week or every other week. (Once a month just isn't enough.) If you change places and times, it will become so confusing and people will start showing up the wrong day and wrong time at the wrong place, and soon everyone will quit.
* Someone needs to be in charge. This is why critiquing classes are good, because there's someone in charge: the instructor. Someone has to be the bad guy or girl, set the rules, man (or wo-man) the timer.
* Speaking of the timer....generally you will have three readers a night. They should send their work out a few days in advance of the meeting so everyone can read their work before coming to workshop. If you have a group of seven to ten people, give everyone two minutes to critique and at the end, two minutes to the writer to respond. The writer should not talk during the critique, nor should anyone else talk while the person critiquing has the floor. When the timer goes off, the person speaking should wrap it up.
* Screen people who want to join. This might mean a committee of three or so people read their work and also know their personality and how they'll fit in the group. You know that cliche....one bad apple..... Writing you want to read is as important as personality, and personality is as important as the writing quality. A stellar writer with a lousy attitude will ruin the group.
* I wouldn't recommend just having an open group. You need the same people meeting regularly, which forms a bond and creates trust. If you meet at a bookstore, most likely you'll have to let anyone into the group who wants to be in. Better to meet at someone's house or find a room in a library or bank. Or get a restaurant to give you a room during the evening where you can buy food and hang out for two hours discussing and eating....
I hope this is helpful. I wrote a story for Poets & Writers some years back about the Fictionaires, a writers group I belonged to for a time. I'll see if I can find it and post it on my Web site.