Set a default sharing restriction for Google+ content
You can control whether your users' posts can be shared outside your organization’s G Suite domain by default. If a post is restricted, people in your domain who receive it can't share it with anyone outside your domain.
If you restrict sharing posts, other Google+ content is also affected. New communities are restricted to your organization’s domain by default, and users' +1 notifications are always restricted
To restrict user content to your organization’s domain by default: