1) The document sharing feature works by utilizing a service provided by Jive. Once your Clearspace instance is configured to connect to that service then end users can share the document (in the right sidebar when viewing the document) by providing an email address for the other person. The other person receives an email that invites them to collaborate on that document through the Jive service. Once the other person navigates to the document they can view and comment on the document.
2) As a system administrator you should see a "(customize)" link inside the "All Content" tab (when viewing that tab) on the home page. Click on the "(customize)" link and you'll be able to change the default for everyone.
Let me know if you have any other questions.