Before revision:  We show that it is the only aggregation function that gives tie-breaking rights to agents who join a group and satisfies an independence condition in the spirit of Arrow's independence of irrelevant alternatives as well as four undiscriminating axioms .
After revision:  We show that it is the only way to aggregate preferences that is monotonic in the agents' preferences and ignores preferences over redundant actions. The monotonicity condition ensures that an agent who joins a group gets to break ties in her favor if differences in preferences are due solely to differences in the utility functions .