VoiceThreads can be shared in a variety of ways.

By default, the VoiceThreads are only viewable by the creator of that VoiceThread.

VoiceThreads can be shared with an entire course, a group of people, or individuals.

In the process of creating VoiceThreads you will be asked if you want to share the VoiceThread with the course.  However, if you need to come back to it later, or need to share the VoiceThread with one specific person, you will need to access the sharing area directly.

Accessing the Sharing area of a particular VoiceThread:
https://voicethread.com/howto/accessing-the-sharing-area/

Sharing a VoiceThread with a Group (VoiceThread calls courses Groups - this means you are sharing a VoiceThread with an entire course)
https://voicethread.com/howto/sharing-with-a-group-3/

Sharing a VoiceThread with an Individual (this is helpful when working on a VoiceThread with another person as part of a group project)
https://voicethread.com/howto/sharing-with-an-individual/