Setup User vs Blog registration

Hi,

I'm new to WPMU but not to WP. I need to create the following setup. I'm currently using the edublog theme and have installed the Pay to Blog plugin.

I'm creating a community of buyers and sellers. Buyers can sign up as users only and sellers can create a blog.

What are the specific links to:

1. register a user
2. register a blog

Thanks.

- Lina.