There is no easy way to do it, in fact no way at all that i know of to integrate forum users and Wordpress users in a single sign on database. As you can see with my site i have a forum section which you can register for which is totally seperate from the blog registration. Kind of annoying, but people only really need to register for the forum so they can post in it, so not a big deal. If you dont mind your users having to register for the blog and seperately for the forum then it's an easy thing to set up.
I use and recommend Simple Machines forum (SMF Forum) which is open source and feature rich. Really easy to customize and very user friendly. It can also be installed via Fantastico which makes it really easy for non-techies
Once installed you would just create a hyperlink to the forum homepage and change the forum appearance theming to match your blog site.
If you need any further info let me know.
