EU Nationals do not need a visa to enter Greece, but a valid ID card or passport is required.
Non-EU nationals in general require a visa for stays in Greece. A visa is not required for semi-annual visits of up to three months for nationals of those countries for which the European Community has abolished the visa requirement. Nationals of countries which do not have a visa-waiver agreement with the European Community must obtain a visa. Detailed visa information is provided by the Ministry of Foreign Affairs. The list of different countries and the visa requirements can be found here: link.
Participants that need an invitation letter to apply for a visa should fill the form.