Subsite register directing to main site?

Heeeeelp!

I am a bit confused by this one.

I have WP multisite installed using subdomains. I want to allow users to register on the subdomains as they are individual to my clients.

They would need to register to buy products from a marketpress store for example.

However, when clicking the register link, users are redirected to the main site and do not appear to be registering on the subdomain.

Please refer to the meta box at http://sandridge.schoolinformer.com

I am sure I must have overlooked something somewhere but have no idea what this could be. I always believed that the subdomains acted independently of the main site.

I am due to demo the site to the school on Friday - please help me work this out.