Membership was by invitation and required the undertaking of a very formal initiation ceremony in front of the other members. Two naked ‘helpers’, or young women would assist the invited candidate to a small ‘closet’ where in private they would sexually arouse him. In an excited state, he was then escorted into the full gathering where the ‘sovereign’ would instruct him to lay his manhood on the ‘testing platter’, an erotically-engraved silvered plate.
Source: Oh Nothing, Just an 18th Century Masturbation Society