as auxclass has explained blogs aren't forums and they aren't wikis. There is no forum that you can embed into a free hosted WordPress.com blog. There is no FTP access and we cannot install plugins on free hosted WordPress.com blogs.You have three options:
(1) Use the P2 theme which is a Twitter-like theme on your current blog.
http://theme.wordpress.com/themes/p2/ and add official users to your blog.
(2) Create another blog and use the P2 theme on it and then create a custom menu on your main blog and include a custom link to the second blog in it. Then add official users to that blog.
(3) Register a forum on the internet and then create a custom menu in your blog and include a custom link to the forum in it.
(4) Are you aware of the meebo chat widget > Appearance > Widgets
(5) As the Admin of a blog you can insert images into your comments and your visitors can include links to images in their comments for your to moderate. WordPress.com allows you and your readers to embed YouTube videos, tweets from Twitter, and PollDaddy polls directly into blog comments by simply adding a single URL into the comment text. http://en.support.wordpress.com/comments/embeds/