One of the other things you need to decide once you have a writers group, is how often to meet and how you're going to do the critiques.
The group I'm currently in meets every other week (or, at least twice a month.) I've been in workshops that meet monthly, but one thing we discovered is that if, for some reason you have to cancel, it's a LONG time until the next meeting. This can be especially bad if you have novelists in your group. The memory can be quite strained by trying to remember who, what, and where with such long gaps between sessions.
In fact, I once had an argument about whether or not you could effectively critique longer works, like a novel, in a group. Wyrdsmiths is mostly novelists, and we do very well. I think, though, that one key component is how often we meet. We started out of a weekly class and almost decided to meet weekly, but thought that might be impossible for those of us with full-time jobs (which was all of us at the beginning.) Twice a month works really well for us, and when we have to cancel or when people have to miss, it's not so terribly overwhelming to try to catch up.
But monthly can work, and may be the only option for people with supremely busy lives.
There are many ways to run a workshop/critique session. I attended one where you showed up with something to read, read it out loud, and people gave you instant, verbal feedback. I found this to be somewhat problematic, because, IMHO, it tended to favor good readers. Thanks to a background in theatre, I can read out loud pretty well. And the listeners noticed that, rather than the flaws in my text. However, you can get a good sense of first impressions this way. Also, if the group membership fluctuates wildly (this was billed as an open house style group), it can work just fine. There are also genres, I think, where this could be the preferred method, like poetry, for instance.
The way we do critique in Wyrdsmiths is that we come with a copy of a hand-out for every member. They take it home to review it next session. With paper in hand, critiquers tend to give a more detailed response. They correct typos, grammar, etc., (though my group doesn't dwell on that, unless the typo is particularly funny, as in the one I once wrote about "quacking aspens.") Since our group came out of a fiction writing class, everyone is very skilled at the art of critique and many of us still follow a structure set up by the instructor wherein we write our first impressions, strengths, weaknesses and then final impressions. (Note: starting with strengths is a good way to keep the author listening, and feeling good about what's to come.)
If people in your group are unfamiliar with how to critique, I offer this great site: How to Critique Fiction by Victory Crayne.
Groups often have other rules, like: "no cross talk" or "the gag rule." The first means that each critiquer gets the floor, no interruptions. The second means the author is not allowed to defend her work (or ask questions, except for clarification,) until everyone is finished critiquing her piece. At Wyrdsmiths we've been together so long that we no longer strictly enforce either of these, but they can be helpful when you're first starting out.