How do users sign up for pro sites

Hi there,

We are currently moving our network of takeaway sites to WPMU and I had a question about Pro sites.

We offer to levels: monthly and yearly contracts. I just set up pro sites with the hope of having these two levels available on a sign-up page that one of our sales reps could sign up new clients on.

My question is this: Is it possible to have new clients sign up using pro sites (or any other of your plugin(s)) if they don't already have an account?

Thanks!