1. A computer network comprising a plurality of local components, a group of which interact by passing messages between them in order to perform a distributed application, in which one or more of said computers comprises: a local-component-hosting execution environment arranged in operation to execute one or more local components of the distributed application; a group membership token handler in a separate execution environment including a group membership token store storing 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, said group membership token handler being arranged in operation to intercept one or more message transmissions from a local components to other computers in said network, to find whether the message-sending local component is entitled to include a group membership token asserting membership of the group with the message, and to include the group membership token with the intercepted message if said local component is entitled to the group membership component; a group membership controller providing an interface that enables a token authority to update said group membership token store in order to change which local components are entitled to have a group membership token including with one or more messages.