Did Jews believe that other peoples belonged to their spiritual community?
The Jewish nation constitutes a "closed" spiritual community. This means that they believe that they have been chosen by God for a special mission, and that this mission comes together with a set of 613 obligations (commandments, or "mitzvot" in Hebrew) that only they must observe. For example, the prohibitions of eating pork and refraining from work on the Sabbath are obligatory only for Jews; similarly, the obligations to study the Torah and to eat matza (unleavened bread) on Passover are for Jews only.
Still, any human being may convert to Judaism if he or she displays a sincere desire to live as a Jew.
Furthermore, Judaism has a feature which few other religions posess: it has a "program" for those who are "outside" of its spiritual community. A non-Jew can merit eternal life by observing the Seven Noachide Laws, which include the rejection of idolatry, refraining from murder, theft, and sexual immorality.
Judaism presents this "program" because it recognizes that all humanity belongs, in a sense, to one spiritual community, which consists of all the descendants of Noah. So, the answer to your question is both "yes" and "no." Jews believe that they form a special spiritual community within the larger spiritual community of humankind while historically accepting converts and providing for nonconverts.
In the faith of Judaism from the original days, the belief was that Jewish people were only considered to be Jewish if they had descended from the original tribes, although from as early as the time of Abraham there is record of converts to Judaism. The lineage was passed down from the mother. The Talmud forbids intermarriage and therefore would not accept others into the faith by conversion in the strictest sects of Jews.
However, with the changes and less Orthodox sectors having been established, people can go through a strict process and convert to Judaism. The belief is, then, that the person would be awarded the same spiritual rights as other Jews.
In the event that there is an intermarriage between a Jew and a Gentile, the child that is born to a Jewish mother is considered to be Jewish as will the daughter's children be Jewish. The son born of a Jewish mother would be Jewish, but his child would not be considered a Jew if his father was a Gentile.
The Reform movement in Judaism made it possible for other non-Jews to convert and for children born of any of the parents, male or female, to be accepted as a Jew.