Where to configure the default user group for file/directory creation?
I don't see a way to do this. Since network accounts have their groups managed by the network account server, it doesn't look like this can be overridden locally. And since the user is part of the network account server they are technically not part of a local user group.

As a test, I used Advanced Options to modify my network user to group 80 (local admins group). Upon reboot my user belonged to the AD group again.

If you want to change group ownership, you will probably need to use the chown user:group functionality in the Terminal on each file or folder you want to modify ownership permissions over.

Another option, of course, is to work off of a local user account and use your network credentials when accessing network resources. You will lose awesome things like single-sign-on though.