You can easily add new authorized users (authors, contributors, admins) via the Dashboard.
Inviting Contributors, Followers, and Viewers
As for adding more than author to a post, see here for a plugin that can do this:
http://wordpress.org/support/topic/how-to-add-multiple-authors-to-a-post
And, this article has a more complete list and guidelines for multiple authors in general:
10+ Must have Plugins if You have Multiple Authors in WordPress
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Also, it is imperative that your WP blog be current and have all updates installed.
Plus there are several plugins that are important to detect malware that might have crept in during time periods when older WP versions had vulnerabilities (since patched with v 3.4.2).