In the choice activity, if you change the setting "Publish Results" to "Always Show results to students" then the results are visible to everyone, students, guests, Google etc (Anything that can view the choice activity). There doesn't actually seem to be a way to restrict the results to a group of users. This has been reported in the tracker as MDL-50292.
I've proposed a patch on that issue (MDL-50292) that would add a "mod/choice:viewpublishedresults" permission to the choice activity to allow us to control which users are actually allowed to see the results.
I'm hoping to get some feedback from other moodlers on whether this is a good idea.