On this page of the guide to Disco Elysium, you will find a walkthrough for the Find out who is in the Union box quest.
Your objective in the Find out who is in the Union box quest in Disco Elysium is to talk to Garte about the paper that was hung on the cafeteria's door.
The manager of the cafeteria will tell you that people from the Union are gathering in the cafeteria. The door to this room will be opened on the next day. Inside, you will meet Titus (the leader of the group) and his men (the Hardie Boys). Talk to Titus to complete the Interrogate the vigilantes quest.