According to Official Guinness Records,
The largest Sirtaki dance was achieved by 5,614 people in an event organised by EKPOL, the Social and Cultural Council of the Prefecture of Magnesia, in Volos, Thessaly, Greece on 31 August 2012
The record attempt started with 6,400 people. The event was organised by Costas Agorastos - the Prefect of Perfecture Thessaly.
For a complete list of 2012 records, please visit 2012 Guinness Records in Greece.