I wish to modify the script that sets up the user groups in order to change them to suit my use of Karoshi. Which files contains this information?

To change the groups that are created you will need to edit:

/opt/karoshi/serversetup/xen/scripts/setupsamba

/opt/karoshi/serversetup/xen/"useful scripts"/generateprimarygroupinfo

You may also have to edit:

/opt/karoshi/serversetup/xen/"useful scripts"/addnewuser