1. In a system having a plurality of accessors and at least one resource, a method for determining accessor entitlement to a resource in response to an accessor request for access to the resource, comprising the steps of: a) associating with the resource an entitlement expression, the entitlement expression including a reference to at least one membership map having membership information for the accessor, the at least one membership map being a bit map; b) associating with the accessor a unique identifier, the unique identifier acting as an index into each membership map; c) evaluating the entitlement expression for the resource to determine the entitlement of the requesting accessor to the resource, the evaluation including looking up the accessor's membership information in the at least one membership map using the accessor's unique identifier.