There has been a significant amount of discussion and work (People Service, XFN, and FOAF to name a few) around the concept of managing personal or social relationships. These relationships are managed from a user-centric perspective and are really a one-to-many relationship between the user and the identities being managed. This allows the identity of the user to be implied in regards to the other identities and groups (e.g. it is my buddy list). The group structure provides a form of role classification that can then be leveraged for permissions and access control among other things.
However, managing relationships between identities at the service level is a little different. For a service provider the relationship is not one-to-many but rather many-to-many. For example, a hospital will want to manage which patients are assigned to which doctors. A doctor can have many patients and a patient might have many doctors. The existing social relationship mechanisms don't really fit this model as these relationships are defined from a service provider centric perspective. The hospital manages the relationships not the user; though the user is free to leave one doctor and move to another.
Where social relationships tend to be defined in a uni-directional manner (from the user to the other identity), service based relationships are often bi-directional. I don't see this as a problem as it is rather easy to model and implement. However, it is important to distinguish between these two kinds of relationships.
Tags: Identity Relationship, People Service, Social Networks