Hi Jay, 

The sharing feature is restricted to users on the same domain. We get this request from time to time, but SmarterMail and domains are significantly sandboxed. Allowing authentication and permissions and security to cross domains would be a huge undertaking for a small number of customers and with added risk. 

Right now, the only way to share contacts outside of a domain is to export them to CSV or VCard. For allowing a third-party program to make changes to your contacts, you would need to utilize the API to create an integration. 

I hope this helps!