Group living animals perform behavioural switches in synchrony to keep the group together. Synchronization cost resides in colluding to a communal switching time rather keeping individual ideal times. We model this situation, assuming that ideal times vary quasinormally and grouping benefit increases linearly with group size. The most common optimal solutions are full synchronization with the group staying together and zero synchronization with immediate dissolution. Partial synchronization occurs only at a narrow stripe of the space. Synchronization cost never causes splitting of the group into two. Stable solutions dictate lower degree of synchrony and lower net benefits than optimal solutions. Under repeated synchronizations, groups stay together or dissolve, unless the animals assort themselves into a smaller group with less variation in the ideal times. The model predicts that larger groups may be more stable than smaller groups.