A writing group is a bunch of writers who read what each member is working on and then get together and talk about it. Writing groups give writers some things that are otherwise hard to get: urgency, encouragement, criticism, and people to bitch to.
If five other people are eager to read your pages, you'll write them sooner.
Praise from fellow writers will probably mean more to you than praise from your mom.
Your fellow writers know what you're capable of, and they won't let you get away with so-so work. Also, civilians may criticize the wrong thing. Writers can pinpoint exactly what needs fixing.
People to Bitch To
When one producer mistreats you, it's humiliating. When packs of producers mistreat all of you, it's a bunch of funny stories.
Who Should Be In Your Writing Group?
No one should be in the group who isn't writing. Girlfriends, colleagues, friends who are kicking around the notion of writing something one day: these people, while no doubt fun to go to a movie with, are not doing what you're doing. Their criticisms may be on the money, but they are readers' criticisms, not writers'. Also, if someone's in the group who's not a writer, then he doesn't ever get criticized. It's much easier to take criticism from someone you tore into last week, and you're going to be that much more precise in your criticisms if you know he's going to get his turn to tear into you.
You don't have to all be screenwriters, but everyone should be at roughly the same level of craft. In my writing group, the rule was that everyone had to be a professional writer, so we had a TV writer, a comics writer, three novelists and a feature film writer. You don't have to be friends already; if things work out, you'll probably become friends.
How to Find Fellow Writers
Most writing groups probably start with a few friends who are writers, then add friends of friends. What if you don't know any writers at your level?
If you're taking a writing class, seminar or workshop, ask some of your fellow students whose writing you like if they'd be interested in working together after the class is over. Whether or not you're a student, you can always ask writing professors if they know any students in their classes whom you might contact.
Put up a notice on at the local coffee shop where writers hang out. You know the one: there are always a couple people sitting alone, nursing their one cappucino for hours, writing. In the notice, describe what flavor of writing group you want to start. You can also put up your notices at the local art house movie theater, the film fan video store, the English department at a local college, your bookstore, and so on.
It's a long shot, but check out the various Internet newsgroups (rec.movies.*), chat boards,web sites and so forth. Post a message asking if anyone in your home town wants to join a writing group.
It might be a good idea to meet with each potential member yourself, alone, before letting him or her into your group. That way it's "your" group, and you also can screen out anybody who might make the rest of the group uncomfortable...