Hi Wild Bucky,
Login capabilities or user specific private content as part of a Content Management System are not features currently available in Muse. CMS features will not be part of the Muse 1.0 release. We intend to add CMS features to Muse in the future that will work with Adobe's Business Catalyst platform. Business Catalyst already has CMS features (see http://www.businesscatalyst.com/product-tour/overview for more info), but Muse isn't yet able to take advantage of them natively.
The integration of Business Catalyst features in still in the roadmap phase, however you could anticipate that adding features in the future will not destroy anything you had already created in your Muse site. Some advanced users have been able to integrate Business Catalyst features in Muse sites today, but we do not have a seamless workflow for doing this in the current release.