In a dynamic coalition environment, organizations should be able to exercise their own local fine-grained access control policies while sharing resources with external entities. In this paper, we propose an approach that exploits the semantics associated with subject and object attributes to facilitate automatic enforcement of organizational access control policies while resource sharing occurs among coalition members. Our approach relies on identifying the necessary attributes required by external users to gain access to a specific organizational object (or service). Specifically, it consists of extracting user attribute sets that semantically match with the attributes of the objects for which a role has permissions. This relies on a closer examination of why a user is assigned a specific role. These attribute sets are first pruned based on their significance in characterizing a role, which are then checked against those submitted by an external user to decide whether to allow or deny access to the specific object. While our goal in this paper is to support coalition based access control, the proposed approach can also aid in automating the process of role engineering.