In EU, it is allowed to perform VAT-less transactions in many B2B and some B2C cases.
Unfortunatelly, as a conference, we are not eligible to provide you with these kinds of exemptions as part of the other side of these regulations.
Where is it written?
According to the European Commission's Taxation rules, B2B services in respect of admission to cultural, artistic, sporting, scientific, educational, entertainment and similar events will be taxed at the place where those events actually take place [Article 53 of the VAT Directive].
If you search for "Example 35" in the above source, you will also read specifically:
For a conference organised in Stockholm for participants paying a fee to attend, Swedish VAT is charged.
For more specific information, you can refer directly to the VAT Directive of the European Commission (offered in multiple languages). The article in reference is Article 52.
So, what can I do now?
For EU companies:
Even if we are applying VAT charges according to EU regulations, you are still able to recover the VAT from your own country. The process varies among countries of the EU and for more information, it is always better to ask your local Tax Administration or your accountant. They will certainly provide you with the latest details. However, some information can be found online as well. The main idea is that you will have to refer to your Tax Administration which will request the VAT refunded from the Greek state and then return it to you.
More information on the requirements, dates and contact points for the Greek Tax Administration can be found at https://ec.europa.eu/taxation_customs/sites/taxation/files/docs/body/el_en.pdf and additional general information about VAT refund can be found at https://ec.europa.eu/taxation_customs/business/vat/eu-vat-rules-topic/vat-refunds_en
For non-EU companies:
For companies outside the EU, currently, the VAT refund is also possible. However, different countries have to sign an explicit reciprocity agreement with the EU country in question in order to make that possible. Due to the amount of information and agreements as well as legislation constantly changing, the best way to get the latest updates is to refer to the Tax Administration of your country.
On the EU side, information about non-EU companies can be found at https://ec.europa.eu/taxation_customs/business/vat/eu-vat-rules-topic/vat-refunds_en as well.