We examine the problem of supporting access by collaborations of scientists to Grid resources. Our aim is to support dynamic collaborations, by which we mean collaborations that can be easily formed and easily dissolved. We argue that current technology for creating Virtual Organisations has difficulty meeting this requirement. We propose an alternative structure, called an Alliance, based on a separation of the mechanisms for forming collaborations of people from the mechanisms for allocating and integrating resources in a Grid infrastructure. The key tools for achieving the Alliance are, firstly, a security architecture and, secondly, an ontological approach to the defining of roles and processes within an organisation. An example is presented to show how these tools can be implemented without needing to change current existing Grid middleware. © 2008 Elsevier B.V. All rights reserved.