Membership permissions to view individual multisites

Hello,

I'm building a multisite network where each sub-site requires a different membership (each site is an "ebook" of sorts).

What is the best way to set up memberships so that:

1. A unregistered users arriving at a sub-site will be directed to register at the main site

and

2. After registering, a user will be granted access to all of the content of the sub-site.

I have multisite set up successfully, but I don't see how I configure the membership access levels to apply to the sub-sites.

Thanks,
Matt