While Maryn's thread
Guidelines to Forming an In-Person Critique Group is geared specifically about what it says on the tin, I'm guessing a lot of the suggestions in there would be applicable to all kind of writing groups. Just that talking about writing groups in general adds another layer of stuff that have to be sorted out to have a compatible group. Well, at least as far as I understand it, Writing Group can mean quite a few things.
Personally, I'm still at a stage where I'm trying to figure out what I think I would get out of a writing group, and really don't have a clue what I can bring to the table myself. For example, I would love to have more people to bounce ideas with, but I don't know if I would be as enthusiastic about bouncing others ideas. Maybe I am, but maybe I'm not. If not, then there would have to be another reason for other people to want to have me in the group. One of the many reasons to for me to join this forum, and joining in on NaNoWriMo, is to figure things like that out.