It all depends on what exactly you are doing.
One site will be the first to register users on. If you are allowing users to register themselves without any plugins this will need to the the main site (e.g. blog 1) for the Network (Multisite) installation. Then you can add the user role to this first site as necessary (or even multiple roles with plugins like [user-role-editor])
The second Wordpress site on the Network can allow users, who have an account on the first, to seamlessly have an account granted on the second site granted with one role given. plugins like [join-my-multisite] allow for this by granting one role (e.g. subscriber).
If you want more complex arrangements, like allowing public to register accounts themselves and not using the Primary site (e.g. blog 1) for user registration use plugins like network-subsite-user-registration.
You can then join the first site users and roles with a secondary site (or more) from the same Network (Multisite) and grant roles (or capabilities) on these sites based on the first site. Effectively you don't control the user access on the second site you control the access for everything from the first site. This can be done using user-upgrade-capability and reduces admin work.
but either way Method 1 or 2 above give you some suggestion.