Mike Harris assures me that he gets a field labelled:
Groups: Only users in all of the selected groups can view this bug. Leave all
boxes unchecked to make this a public bug.
which can be used to restrict access to a bug.
On further investigation, I see that I do in fact have a groups field if I file
a bugzilla bug (or even a RHEL3 bug) but not if I file a RHEL4 beta bug. And I
still don't have access to the "Red Hat Development" groups checkbox.
Not currently a member of any groups so therefore you will not see a group list
on a bug entry page. If you see one on a bug edit page, it is checked to that
group by someone else and it should be uneditable.