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, you must share it with them from your own Pro Educator account. Since you always have editing access to student work, you are always able to share it. With your Pro Educator account, you are able to share with anyone in the world, including non-Ed.VoiceThread members.
Learn more about secure sharing here.