In order to share a List externally, 'Publicly Viewable' must be checked in Create/Edit List.
When you click the Share List icon in List Management, you will be prompted with a public URL that you can copy to share with clients or other external parties.
If Access Code is required to view list, clients will be prompted to input the set Access Code, their Name, and Company before viewing the List. Otherwise, the list will be publicly accessible to anyone who has the link.