First backup your /etc/group file with;
cp /etc/group /etc/group.SAVE
from the terminal,then open /etc/group file with the text editor.The first line will be root's group,just delete any user you don't want it there and save the file.
For example,my line looks lke this;
so if I want only root to be in that group then I would delete alan and the , and then I would save that file.Done.