[support] Sharing 'users' table with another database?

Has anyone here know how to get Drupal to share the 'users' table with another database? (Or, in other words, connect to a different database to access the 'users' table.) I have a "people" database that I share between all my different projects, so that all of my clients/users need not sign up multiple times. Signing up to anything gets them signed up to everything. Change their name/email/password once, it's changed everywhere. I've been able to do this because I was hand-coding all of my projects, but now I'd like to use Drupal, yet still access this central "users" table (I can make it exactly fit Drupal's specs/schema). Perhaps there's a different way to achieve what I'm trying to do? Any suggestions or pointers? Thanks much.