I don't know about groups but...

Not following the rules of nanowrimo (like having written part of it before hand) just puts you in the rebel camp. While the site technically says you should start at word 1, there are others who don't always follow those rules (among others). They even have a forum: http://www.nanowrimo.org/en/forums/nano-rebels


Also, for a personal plug. There is this little group on WDC that allows for people to work on started novels and to pick their own word count goals.

GROUP

NaNoWriMo Plus  (13+) A writing group inspired by NaNoWriMo with monthly goals. #1593387 by Dawn Embers

I say, if it gets you to sit down and write the first draft, who cares if you have 5 chapters done before hand. But that's me. I usually start at word one and if I have a scene written beforehand (as I sometimes write random scenes for contests on WDC) I do a total rewrite when adding that part in to the novel anyways. But we all have our own approaches. You have to pick what you want to do.

Hope this helps a little at least. Good luck with the group/review search.