Sharing options allow you to define who can create, use, see, and manage the groups created in a domain:

The first section will allow you to define the default access to groups for people outside the organization. If you select Public on the Internet, anyone will be able to view, search, and post on the groups, which is great for interacting with customers and building a community around a product or service. Selecting Private will restrict access to people within the organization only—external users would require an invitation.