In a modular on-line virtual environment, in which each module of the on-line virtual environment has a plurality of parallel instances each able to host a limited number of users, a server arranged to administer the modular on-line virtual environment comprises a network communications arrangement operable to receive data representative of the actions of users within an instance of a first module of the modular on-line virtual environment, and to transmit to each user within that instance of that module data representative of the actions of each other user within that instance of that module, behavior analysis means operable to analyze user behavior within that instance of the first module, in which if the behavior of two or more users satisfies a predetermined criterion then the two or more users are classified as belonging to a social group comprising the two or more users, module instance selection means operable to select an instance of a second module for a user when that user moves within the modular on-line virtual environment from the first module to a second module, in which the module instance selection means is operable to select an instance of the second module that has the capacity to accommodate the greatest number of the common social group once one member of that social group moves within the modular on-line virtual environment from the first module to the second module and the server is operable to place a subsequent respective member of the social group in that same selected instance of the second module if that member moves within the modular on-line virtual environment from the first module to the second module.