Google Drive, Dropbox, DO Spaces, and other services provide secure online file sharing. However, there are not many such services that integrate well with the user base you have on your WordPress site. ScoutDocs by Mark Jaquith aims to fill that gap.
ScoutDocs is a file sharing plugin that runs on your site, but stores your files securely in the cloud. Files are served via a global content distribution network, and e-mail notifications are sent from the ScoutDocs service, so you know they’ll be delivered.
You can view a short 3½-minute walk-through of the plugin. I didn't mention the email notifications received by users. I also didn't mention that it'd be nice to see some way in which the UI could be more well integrated into the look of your site. Apart from customizing the logo, there is no current way I know to embed the file management interface in the content space of a page.
All in all, it looks like a nice service. I'll be looking to possibly use it for sharing contracts, timesheets, invoices, and other such files with my clients.
? The free version limits file size to 20MB and total file storage to 500MB.
? The paid version (at $19/mo) allows files up to 50MB and total storage of 5GB.