。
The sentry made everyone say the password.
Literal
Sentry [topic-は] everyone [to-に] password [obj-を] made-say.
The causative: 言わせた from 言う + ~せる. The causee (the people made to speak) is marked with に — signaling a neutral 'made/had them do it.' 合い言葉 ('password, watchword') is a compound from 合う + 言葉 ('matching word'), originally used in military or secret-society contexts.