Forum User Creation

Hello all,

Bit of a n00b question (I think)
I've got wordpressmu 2.8.1 installed with the forums plugin running. I've got the forum up and running and it looks great, but how do I allow the random user that comes by the site and clicks on the forum tab the ability to post?

I assume they'd need to create a user name, but how can I put this on the forums page? I have wordpressmu installed with subdomains, so I'd like for the users for the forum to only be associated with this particular subdomain and not the entire wordpressmu site. Is there a way to do this?

Thanks in advance!