Participants from Schengen Countries, USA and Japan do not need a visa to travel to Greece. For US and Japanese citizens this is valid for up to a 90-day visit. Participants from other countries should consult the Greek Embassy of their country on whether they need a visa or a letter of invitation to apply for a visa. Please also check the following http://www.mfa.gr/en/visas/visas-for-foreigners-traveling-to-greece in order to find information for VISAS for foreigners travelling to Greece.
If an invitation letter is required, participants should contact the Local Organizing Committee. Please allow sufficient time ahead of the meeting as the letter of invitation and the overall application process can take time.