If you are not a national of Switzerland or a country in the European Union, you may need a visa for attending the workshop and the subsequent DUNE collaboration meeting in person.
For more information, and to get an invitation letter for a visa, please refer to the following pages:
https://neutrino-secretariat.web.cern.ch/invitation-letters
https://usersoffice.web.cern.ch/visa-requirement-and-procedure
https://usersoffice.web.cern.ch/sites/default/files/pdf/Formalities/VisaRequirement.pdf
Please note that you may also need an additional document called "Convention d'Accueil".