如何从Wordpress外部登录Wordpress（mu）用户？

I'm working for a company that is using Wordpress MU to supplement other functions on a member site. We have a user signup process that creates the appropriate Wordpress MU users and blogs on signup, but I'm having real trouble figuring out how to log someone in to a WP blog from outside of Wordpress itself. The documentation these methods seem to be non-existent or just too obtuse for me. I'm working for a company that is using Wordpre