Users automatically assigned to all blogs

Hey,
I'm trying to do something a little bit unusual with a WPMU install. I have a WPMU that I will be using for community news websites. I intend to have contributors add blog posts to one blog and then have all their posts propagated out to all the other blogs probably using something like Autoblog. (Some content will be centrally pushed like this, some entered on the individual blogs for local stuff)

Anyway, I'd really like the author bios to carry along with their posts so presumably they need to be users of each site on my WPMU? It will be a major pain if every author has to be added to every blog as it could run to hundreds of blogs eventually and probably several dozen authors.

Any quick way of having authors on one specific blog automatically added to all the others? Any suggestions greatly appreciated.