assembly room 
noun chiefly British
A public room or hall in which meetings or social functions are held: [in names]: the Derby Assembly Rooms...
- It drew laughter from the other schoolchildren packed into the assembly room.
- Meriod Hall was a great assembly room in the centre of town, a large, fine space with glossed wood floors, and brightened by chandelier light.
- The first floor would be used for training and the ground floor would have an assembly room, lounge and refreshment bar.