Students are always able to share VoiceThreads with their Groups and any contacts who are members of the Ed.VoiceThread network. They are not able to invite non-Ed.VoiceThread users to comment on their VoiceThreads. This means that if you want non-Ed.VoiceThread users like parents or community members to comment on a student-created VoiceThread, an educator must share it with them from his or her own account. Since educators always have editing access to student work, they are always able to share it. With a Pro Educator account, they are able to share with anyone in the world, including non-Ed.VoiceThread members.
Learn more about secure sharing here.