I'm going to stay away from Sharepoint. We're all Macs here, and so we're naturally suspicious of Microsoft products.
I've looked at the basic features of Basecamp and it looks like it might work. I'm going to take the "Tour" and watch a few demo videos, but it has what we're generally looking for. The trial will be most telling.
I have another issue still: We all need to have access to the same external hard drive (to access such things as press releases, PDFs of books, book cover images, admin files, etc.). I understand Basecamp has fairly decent storage, but it's not enough to hold our entire digital archives. Does this require setting up a server? We've tried the various file-sharing features in Mac OS X, but we've never seemed to get it to work. Is it possible to access remote computers without setting up a server? I've never really known how these things work.