We all know about britian leaving the EU, but when a country joins the EU, is there an offical word other then join?
It's either joining, or becoming a member. https://europa.eu/european-union/about-eu/countries/joining-eu_en
you could make a word up -like Brexit but in reverse, so if you are say talking about turkey joining it could be "Tentry" or "Turentry" ,or Tadmision