To find the infamous Guildhall in Baldur's Gate 3, you'll arrive in the city of Rivington and begin Act Three. I'd call this the halfway point of the game, and it's here that you'll receive a new map to help you find new locations, like the Guildhall.
Return to the Guildhall Location – Baldur's Gate 3 PS5 Console Controller
To find the Guildhall in BG3, enter through a door labeled "No Entry – No Exits" in the Lower City Sewers via a passageway beneath the Basilisk Gate Barracks.
As you might expect from a seedy underworld group, there is no clearly marked entrance to the Guildhall. Instead, you have to go through a few different entrances before finally finding one that leads to the entrance where the criminal enterprise is located.
To start, you'll need to have reached the Basilisk Gate waypoint, the first one you'll encounter after passing Wyrm's Crossing and the Wyrm's Rock fortress. For reference, you'll need to make it to the Lower City of Baldur's Gate to reach this waypoint.