In current welfare state analysis there is little theory to explain the action of exclusive groups. This article explores the possibilities of a theory that focuses on the interaction between individual choices and strategies, the formal systems, policies and rules of governments and the informal norms and practices of groups. The argument is that club theory, a branch of public choice concepts, represents a promising new element in such an approach. A theoretical frame has to account for both collectivisation and fragmentation processes - how mass solidarities in welfare states come to be created and in turn to be broken up into narrower mutualities. Migration is used as an example to show how the actions of migrants and policy decisions about them affect these dynamics or are affected by them.