Brandon and Dan do have a writing group, so the assumption that professinoal authors don't use writing groups isn't necessarily true.
Yes, it is helpful for new writers to have others help workshop your story/writing. There are sometimes drawbacks. The writing exuses podcast on writing groups pretty much sums it up:
http://www.writingexcuses.com/2008/11/10/writing-excuses-season-2-episode-5-writing-groups/As for the original writing group of this forum, the Abridged Speculists, that group hasn't been active for at least 2 years. They first got together a loooonnng time ago. Some of them knew each other in real life, some met on this forum, and formed a group that could meet online.
As for group set-up, how often you meet, how you rotate submissions, it really depends on the group itself, those who are submitting, and the purpose of your group. I'm part of the
Type & Gripe, Ink group, and we meet usually once a week, online via IM, and workshop one submission for an hour. All of us are too busy to spend more time than that, so it works for us. We still get our stories workshopped once a month (since there are only 4 of us), but the time commitment doesn't overwhelm us. Unfortunately this winter one of our group is in the middle of grad school so we've only been meeting once a month for 2 hours until she's finished. Then we'll readjust our schedule.
How do you find a group? You can always start your own. You can ask around school/work/church--find people with the same interests are you do, and they may know someone who's a part of an existing writing group.