The trick is simple with one prerequisite. The prerequisite is that one ocument library should exist which can be access by all users of the site.
Upload and open or open an existing MS Word Document. It will have Shared Workspace on the left of the screen. If not, click on “Shared Workspace” under “Tools” menu. “Shared Workspace” select “Members” tab and it has option “Send an email to all members…”. Click on it and get email address of all the users of the site.