At what temperature does the winter cluster recruit all the bees of the colony?

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Correct answer: 3 (~−7°C)

Even though the bees in the winter cluster appear immobile, the cluster changes shape and moves depending on the location of food stores and the temperature of the hive walls, which may be more or less exposed to sunlight. The winter cluster is often in contact with the south-facing wall of the brood chamber and beneath the inner cover, as heat naturally rises. The position of the winter cluster and the arrangement of the bees composing it represent a near-perfect model of energy economy.

As soon as the outside temperature drops below 15°C, the cluster begins to form; it is complete—recruiting all the bees of the colony—at −7°C. The lower the temperature, the more the cluster contracts, in accordance with the principle of convective heat transfer, which increases with surface area. At very low temperatures, the cluster eventually retreats deep between the frame spaces and becomes invisible.

For further reading: Sense and nonsense of hive thermal insulation