1. A computer network comprising a plurality of computers, each hosting one or more local components of a distributed application, wherein said distributed application components interact by passing messages between them in order to perform said distributed application, wherein one or more of said computers comprises: a plurality of separate execution environments as partitions; a trusted computing platform for providing trust services to each execution environment; and a store having instructions encoded therein executable by the computer to cause it to: i) execute one or more of said local components of the distributed application in a plurality of first partitions; ii) in one or more other partitions shared by the plurality of first partitions, a) store one or more group membership tokens for said local distributed application components and information indicating which components are entitled to include the group membership tokens in their messages, b) intercept one or more message transmissions from said local components to other computers in said network, c) find whether the message-sending local component is entitled to include a group membership token asserting membership of the group with the message, d) include the group membership token with the intercepted message if said local component is entitled to the group membership token; and e) provide an interface that enables a token authority to update said group membership token store in order to change whether said local component is entitled to have said group membership token included with one or more messages.