In order to share your exhibit with a designated group, you will first need to publish it. While in edit mode, you will see a "Publish" button in the upper left hand side of the screen:
Choose the radial button for "My organization" and hit "Publish Story." It will take a second to publish. You can then copy the url in the browser, and share it with your Professor. You can revert back to "Private" at anytime.