Build user access based intranet website

I need help in building a website where users can post articles. They can choose these articles to be publicly viewable, or for themselves. They can also invite people to view the articles. The article is them viewable by onl the owner and the people invited to view the article. There would be another section where user can come and start public articles. Another section will list the public articles and enable others to add to the article. In such cases, the article will mark who wrote what and when. (Similar requirement should also be incorporated for user specific area) The website should require registration with activation link Each post should have a security code encorporated to avoind spambots The article editor should be a WYSIWYG editor.