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.