I've read that there's supposed to be an import feature, but I don't see that anywhere, even in the User management view shown below.

http://screencast.com/t/TBZUH9Le5nr 

How are users expected to be added to the forums on an existing site? In looking at the database, I could write a query, but I'm guessing this isn't the preferred way. Right? Ideally, I'd like to pre-assign some permissions to specific customers, employees, and other known privileged people before announcing this forum.

The User Import page you can access from the module actions menu

You can add all existing users via the import function. The user is also automatically added to the forum database when the user access the forum for the first time.

There is also an Scheduler available that runs every day and automatically import all users & synchronizes the roles.

As for the scheduler, does it work off of a delta, or does it parse the entire user accounts table each time it runs?

Also, now that the users are imported, they're all showing as active within the past 30 days and 24 hours. Is that a bug? I realize that they probably just have today's date from the import, but perhaps that should be their last login date instead of today's date when you do the import, so this is prevented.

http://screencast.com/t/6BPhNE7hmfO 

Yes Sorry. In the next version when you create a new board the user & roles will be automatically imported.

I agree, i changed it now for the joined & lastvisit date uses the last visit date from dnn.