Collegia Contra Legem

The Collegia Contra Legem are illegal societies (literally "colleges which are against the law). It is unlawful in the Roman Empire for any organized group such as a guild, club, or society, to meet without the permission of the relevant government authority (the Prefect of a town; the proconsul of a province; or the Emperor). Any guild or organization that meets without such permission stands in violation of law VI of The Twelve Laws of Rome. Violation of this law is punishable by death. In other words, joining an illegal society is a capital offense in the Roman Empire.
By definition, any society whose purpose is to conduct illegal activities, such as a guild of thieves or a society of assassins, would be unable to receive official sanction, and would be be universally considered a Collegium Contra Legem. One need not be present at meetings of such a guild to receive a death sentence -- mere membership (if that can be proved) constitutes a crime punishable by execution. Consequently, membership in a guild that both meets without government permission and conducts other forms of illegal activity, such as murder, is doubly dangerous -- one can be executed for the actions taken, or for mere association with those who take the actions.
For these reasons, most illegal guilds are extremely secretive, slow to trust outsiders, and eschew any signs or symbols of membership (such as medallions, particular articles of clothing, and the like).