We study market segmentation as the outcome of a cooperative game between consumers who interact with a monopolistic seller in groups. We introduce two new solution concepts, the weakened core and stability, that coincide with the core whenever it is nonempty. We show that these concepts are equivalent and characterized by efficiency and saturation. A segmentation is saturated if shifting consumers from a segment with a higher price to a segment with a lower price leads the seller to optimally increase the lower price. We show that stable segmentations that maximizes average consumer surplus (across all segmentations) always exist.