Judaism has been a minority faith group for thousands of years. Throughout much of that time, the majority faith group has been either Christianity or Islam. Both of these religions are offshoots of Judaism and, hence, monotheistic. That is, not recognizing the validity of other gods. However, since Judaism was recognized by both of these daughter religions as the prior, incomplete, revelation of God, they decided to permit an active Jewish presence in their midst. Of course, restrictions were placed upon Jews living in Christian or Muslim society. Proselytizing was one of these restrictions. Jews were not permitted to seek converts, and in some cases, were forced to grant access by the majority faith group's missionizers.
Thus, while we know of converts to Judaism, it was not a large widespread movement and certainly not sanctioned by Jewish leaders. In fact, the opposite has tended to be the case. There is much Jewish material which suggests that Jews "oppose" proselytizing. Before modern times, it would be exceedingly difficult for Jewish leaders in Christian and Moslem countries to advocate missionizing to the host society. This is still the case in Moslem countries.
Today, however, Jews in the U.S. find themselves in a society where free market reigns supreme. This includes the free market of spiritual ideas, as can be seen by the explosion of interest in the New Age Spirituality Movement. Recently, there has been a quiet but significant emergence of outreach by Jewish movements. This outreach has been focused on those about to be interfaith couples, but is open to all. The idea being that Judaism has something unique and spiritually satisfying. Perhaps in cases where non-Jewish partners or others are non practicing of their religion, they might find fulfillment in Judaism.