Single WP user roles and WPMU roles are the same. Each blog has its own administrator each blog admin can add users to their blog under Dashboard-->Users. Each added user can range from Subscriber to Admin.
The difference with WPMU is the users must register at the home blog or be added by the Site Admin to the community first. "Outside" visitors can only be added to a blog after they have visited wp-signup.php.
The roles/capabilities are defined within each blog, not attached to the user. A user can be a subscriber of one blog, an admin of their own, an Author on a few others. Each blog admin controls their own users. The Site Admin only controls who registers at the site.
To sum up, users without blogs can be added to blog at Dashboard-->Users, provided they indeed are registered.