You can not delete this group it is a as a primary default group so there will be no modification for this area in near future. Here is a step-by-step guide below, this will be added to the FAQ.

-create new user
-create new group
-add this user to new group
-change default group of this user (it is his primary group that cannot be removed)
-remove user from users group


Users can be a member of many groups, but the most important is the
default group. If you haven't changed default group from "users" to another
group like yours "group1" you can't remove membership for group
"users" as pointed out above.

Also in file/directory owner only default group will be seen.