Switzerland is the only European country currently present in the top 10 of the TOP500 list of the world's fastest supercomputers. The Piz Daint supercomputer at the Swiss National Supercomputing Centre (CSCS) is listed as the 6th fastest supercomputer in the world.
The countries that are in the EuroHPC Joint Undertaking from the start are: Austria, Belgium, Bulgaria, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Latvia, Lithuania, Luxembourg, the Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia and Spain.
The European Union, represented by the European Commission, is also member.
So the new countries that have joined in the last days are Ireland, Hungary, Norway and Slovakia.
Update 2018-10-02: Switzerland had problems in being a founding member of EuroHPC JU for Swiss legal reasons. However, the country will join as soon as the JU is established.
Getting lost in who joined/signed EuroHPC and when? See: