Support Forum

Simple:Press is an integrated WordPress plugin. It is not that existing users were 'added' it is simply that there is no such thing as a Simple:Press 'user'. Your users belong to the website.

You can prevent users from looking at the members list and viewing profiles of other users by setting the appropriate permissions against the permission sets you choose to use in your forums. Depending on how you assign Permission Sets and User Group combinations in your forums then this need not apply to all your users - that is, of course, up to the structure you impose.

James Mueller said

With the exception of our admins, all of our users are 'authors' -- they are content contributors who cannot access the site.

As far as WordPress goes they are still users. If they have a user account on your WordPress site then they are users - as simple as that. At some point they registered or were created.

So, I guess I'm wanting this to behave like other forums: a user (member) is only registered if he clicks 'register' and provides his credentials.  I'm looking for a way to keep these (and future) authors out of Simple:Press by default. 

And that is the way SP works as I already stated and explained in my fiorst post.

Is it possible to not show 'authors' as SP registered users?

Yes - as already answered above. Perhaps you should look at the Getting Started section of our codex which explains in simple terms how user groups and permissions work.

I see within SP, I have the ability to 'delete' users. But would that delete users from WP, including their posts?

Sorry - but as I have said before and will repeat again here -  there is no such thing as a Simple:Press user. A user is a user on your site, If you delete them - they are gone.