When inviting guests into the Greenroom, you'll have the choice of adding them with two different permission levels.
To invite a Presenter or viewer, click on the image to the right of "Setup," which shows a person with a plus sign attached to them. You'll then get a dropdown with two tabs. Click Invite users to display the field where you can enter that users email.
Presenter invites should be reserved for those that will be on camera. Keeping this mode exclusive to actual presenters will optimize the greenroom experience.
Viewer mode allows individuals involved in the production to be backstage during a rehearsal or show, but without a camera feed. A viewer has access to all the functionality of a presenter including notes, chat and media uploading.
NOTE: Keep in mind that the Presenter or Viewer that's invited must use the same email they were invited with for creating their Greenroom Profile, otherwise the Greenroom won't recognize them and allow entry.