If your users often contact people outside your domain, such as consultants or partners, you can add them to your Directory as shared external contacts. People in your organization can find the profile information for shared external contacts in many Google services, such as when they enter addresses in Gmail. Shared external contacts aren't available in Cloud Search results.
Add external contacts
You can add external contacts to your Directory in 3 ways:
Domain Shared Contacts API—For admins who are comfortable with using SDKs. Use these APIs to add, edit, and delete external contacts. Before working with APIs, you need to enable APIs access. You should also be familiar with protocol basics. Once you're comfortable with APIs, use the code libraries for reference.
G Suite Marketplace apps—For admins who want a user interface that doesn't require programming. Search for “shared contacts.”
External contact limits
You can add up to 50,000 external contacts.
If you have a large organization, add only the shared external contacts that most users need. When only a few users need a particular external contact, either:
ask those users to add the contact to their personal contacts, or
create a Google group that includes the internal users and the external contacts.