register Subscribers and Authors

Hi, I know there is an old post about this (http://wordpress.org/support/topic/332741?replies=10) but no one was able to answer it. Now, nearly a year later I have come across this problem and I cant seem to find a solution.

I need to register two types of users on my website. Ideally I would like two different registrations for them so that once completed they will automatically be registered as a subscriber or author, depending upon which form they register with.

Any suggestions?
Thanks!