The Thieves Guild, otherwise knows as the Thief's Union (怪盗同盟, Kaitō dōmei)? in the Japanese version, is an online feature in Persona 5 that requires an Internet connection. It is accessed by touching the controller touchpad when available in the PlayStation 4 version, and "select" in the PlayStation 3 version.
The Thieves Guild is a service that provides many features to the players. It allows the player to check what activities other players have done on that day, as well as what answer they chose to the questions asked in class and the entire player base's average level for that date. However, it can not be accessed during exams so the player may want an exam guide ready.
The Thieves Guild also offers services during dungeon exploration as well. However, it can only be used up to three times per dungeon visit. A Downed ally may be taken hostage by an enemy, and the player has to negotiate in order to free them. With the Thieves Guild, the negotiation process can be skipped. After a set amount of time during battle in a Palace, an enemy will try to run away, which will increase the Security Level. By making use of the Thieves Guild, the enemy is prevented from running away.
A "Network Fusion" can also be performed in the Velvet Room. During fusion, there's a small chance a skill unique to Network Fusion can be inherited: