Egregore was a collective ritual held among the ancient hunting and agricultural societies. Usually leading the ceremony was a sorcerer or witch doctor.
The ritual was to assure a plentitude of animals for the hunters and a sufficient rainfall for the farmers. In the ceremony the collective energy of the groups attending, which composed their egregore, or magical mana, was combined with that of the leader, or officiator.
Similar ceremonies were universally used for rainmaking. A.G.H.
Source: 2, 85.