The restriction of access is a mechanism by which organisations protect their information assets. Requirements models use actor definitions to describe users and to specify their access policies. Actors normally represent roles that users adopt, while roles can represent different things, such as a position in an organisation or the assignment of a task. Current requirements modelling approaches do not provide a systematic way of defining roles for incorporation into access policies. We address this issue by proposing a framework that facilitates the derivation of role definitions from their wider organisational context. We illustrate how our framework can be used to extend a formal version of i - to define and verify access policies definitions - and demonstrate its applicability via a case study.