In which communities is nusach Sefard used?
A community can be people from a specific place (I.e. Spain), or a certain religious style (I.e. Chassidim).
Nusach Sfard (not to be confused with Nusach Sefaradit Or Edot HaMizrach) is the nusach (that contains many Kabbalistic inyanim) used by families and communities who ancestrally were influenced by the teachings of the Ba'al Shem Tov (founder of the Chassidus movement) and his many students. Nusach Sfard is primarily used by (Ashkenazi) people who originate from much of Eastern Europe (Russia, Galicia, Hungary, etc...), but still maintaining Ashkenazi practices. Most if not all Chassidim use Nusach Sfard (or its many variations). Many non-Chassidim (i.e., those families who do not have a Rebbe in the classical sense of the word, but turn to the Rav/Moreh D'Asrah of their community for guidance) also use Nusach Sfard as well. Nusach Ashkenaz is used primarily by those families who withstood the influences of the Chassidus movement. Geographically, Nusach Ashkenaz was maintained in Germany, parts of Poland and Lithuania.
many chassidim use nusach sephard but are certainly not the only ones who have or do. there are also many variations of nusach sephard. all have at least some influences from the arizal. there is also nusach arizal using today by chabad (more more information on that version look at the introduction in a tehillas hashem siddur). the arizal didn't write down his siddur and there have been many variations over the years not all of which were intended to be used as prayer books and many intended for scholarly studies of kabala. I'm not sure if you can say there are specifc countries where nusach sephard was used in the past as there are many countries that have had communities that have used nusach sephard.