Access Control for Hierarchical Joint-Tenancy
  Basic role based access control [RBAC] provides a mechanism for segregating access privileges based upon a user's hierarchical roles within an organization. This model doesn't scale well when there is tight integration of multiple hierarchies. In a case where there is joint-tenancy and a requirement for different levels of disclosure based upon a user's hierarchy, or in our case, organization or company, basic RBAC requires these hierarchies to be effectively merged. Specific roles that effectively represent both the user's organizations and roles must be translated to fit within the merged hierarchy to be used to control access. Essentially, users from multiple organizations are served from a single role base with roles designed to constrain their access as needed.   Our work proposes, through parameterized roles and privileges, a means for accurately representing both users' roles within their respective hierarchies for providing access to controlled objects. Using this method will reduce the amount of complexity required in terms of the number of roles and privileges. The resulting set of roles, privileges, and objects will make modeling and visualizing the access role hierarchy significantly simplified. This paper will give some background on role based access control, parameterized roles and privileges, and then focus on how RBAC with parameterized roles and privileges can be leveraged as an access control solution for the problems presented by joint tenancy. 
Jonathan K. Adams, Basheer N. Bristow