According to Official Guinness Records,
The most people treading grapes is 977 and was achieved by ACOBASA (Spain), at an event organised by Leticia Alfaya Lozano (Spain) in Labastida, Álava, Spain on 25 September 2010.
Labastida is a village of 1,500 people located in the province of Álava, in the Basque Country, northern Spain. Its main industry is the production of wine. For this attempt 25.000 kg (27.56 tn) of grapes were used and 17.500 litres (3849.46 UK gal) of juice were squeezed.
For a complete list of 2010 records, please visit 2010 Guinness Records in Spain.