Combining multiple user registrations/installs

I’m considering using WordPress for a pretty large newspaper website, but am considering using several WordPress installs, one for each “category” (as I call them, i.e., one for News, one for Sports, one for Lifestyle, etc.)

In doing this, is there a way to combine all the different users? For instance, if someone registers at the “news” install, can they automatically be added to the “sports” install, and so on?

Make sense? I guess what I’m really worried about is creating one HUGE database/site, thus bogging it all down.

Hope this makes sense. Many thanks, all!