Wp2BB integrates WordPress and PhpBB. It automatically creates new topics in your forum for the new posts and/or pages in your blog.
It could theoretically happen if you choose to include the WP post text in the correspondent PhpBB topic, but this is fully configurable in the settings ("Topic message text" option). It is definitely better to create PhpBB topics generic text such as "Discussion about the post 'xxx' in the blog", just using the WP post title
Yes, the "Topic message text" setting will accept HTML and BBCode
No, Wp2BB does not handle users in any way. This feature could be implemented in the near future. In the mean time you might want to check WP-United. Goes pretty well with Wp2BB .