A monthly pairing

Colluders place their people only in mixed pairs, where one is needed — two of their accounts paired together pass with nobody there. Three colluders spend their tokens on one empty account. If an honest person attests it, their default is set to colluder — they are assumed to do the same next time, until they change their mind — the sock is approved, and next round the colluders hold an extra account. If they pass, the sock dies and three colluders invite a replacement.