As of this writing, N26 operates in all the Eurozone countries, except Malta, Cyprus, Lithuania, and Latvia (although the latter two are set to return soon). That leaves Germany, Austria, Spain, Italy, Ireland, Slovakia, Greece, France, Belgium, Estonia, Finland, Luxembourg, the Netherlands, Portugal, and Slovenia. Recently, they made their EUR denominated accounts available in Norway, Sweden, Denmark, Poland, Iceland, Liechtenstein, and Switzerland. They are now also offering local GBP accounts to residents of the United Kingdom.
Since July 2019 you can also open N26 accounts in the United States, and since August 22 you no longer need an invitation to join.