If you want to build your site around WWF then you can simply include the "common.asp" file within your pages.
You can then grab basic user info such as Author ID, Username and Group ID with this variables:
lngLoggedInUserID
strUsername
intGroupID
Also you will need to go into the includes/settings_inc.asp file (not sure on exact name) and uncomment the cookie path so that it is "/".
I have a tutorial on
Member-Only Pages, it should give you a head start.
the API would be more for making the forum work with an existing site I beleive.