Elgg is a white label, open source social networking platform. It offers blog, networking, community, collecting of news using feeds aggregation and file sharing features. Everything can be shared among users with access controls and everything can be cataloged by tags as well.
Elgg is not available via the DreamHost "one-click" installer, but can easily be installed on DreamHost servers using the default DreamHost PHP 5 PHP-CGI configuration and the documentation available at the elgg.org website.
Note that there is one issue while installing. Elgg uses a .htaccess file to check if you run php5, but this fails on dreamhost. You might need to comment out this part from the .htaccess file.
ErrorDocument 500 "Elgg error: Elgg does not support PHP 4." RedirectMatch 302 .* index.php