Pro Sites Newbie

Hi ... I have just downloaded Pro sites and trying to get my feet. I am working through the documentation but seem to have hit a roadblock. My site is a Multisite Buddypress site.

What I need my site to do
* Membership Level A - Pay To Blog - These users only have the ability of creating a new blog. They pay a monthly free (no free blogs). All sites are private and can only be accessed by users that the site owner invites or adds to the site.

* User Level A - {free} These users can create accounts and can access only the main site. These users can then be added to blogs by their owners and therefore write and participate in the blogs.

I think I have got the back-end right (from the video) but I can not see how or where the sign-up page is. Do I need to change the network settings to allow registrations ... but when I do this it just seems to let anyone create a free site.

A bit lost

Thanks for your help