There's a firm out there called A.D.A.M. that supports father's in divorce cases. Let me see if I can find a link...don't know if it's a local thing or not...
http://www.divorcesource.com/MI/pages/mensdivorce.html
I guess it is local, but it says you can call for referals in your area. It's not a group rather a resource perhaps?
I'm sorry you are going through something like that with kids that age. I myself haven't had that problem and wouldn't wish it on anyone. Good luck to you and yours!