There has been a known and recorded Jewish community in Spain for millennia, and when Spain was part of the Roman Empire, Jews generally lived there peacefully and successfully. This began to change in the 5th century, when Rome officially converted to Christianity under Constantine. As Christianity saw itself as superseding Judaism, it was seen as important to undermine them and separate them from the broader Christian community, even as they were preserved as "witnesses" to Christianity (as fellow believers in the now-Old Testament). In fact, preserving yet degrading Jews was a symbol to them, as well as to Christians, of their rejection of Jesus.

At first, under Roman rule, there were doctrinal limitations on Christians relating to their interactions with Jews, and gradually the relationship turned more adversarial and physical.At first, once Germanic tribes, which had already been converted to Christianity, invaded Spain, the condition of the Jews remained similar to what it had been under the Romans. Jews retained their status as Roman citizens with full rights, even as their Christian neighbors were being enjoined to interact less with them and degrade them. While at first the Visigoths were an isolated group who were based largely in Toledo and practiced Arianism rather than trinitarian Christianity, in 587, about 150 years after the Visigoth conquest, King Reccared I converted to Catholicism. Now the Jews were the main outsiders, and their position in the kingdom became even more precarious.

For reasons that were both religious (the zeal of the newly converted) and political (Visigoth Spain was highly fragmented and precarious and persecuting Jews may reinforce alliances with the Church), the policies of the Visigothic kings for the next 150 years created some of the grimmest and most repressive conditions in Jewish history. At first, legal restrictions with religious imprimatur (that could actually be used as precedent throughout Europe) were set in place, such as forbidding Jews from owning slaves (making it difficult to impossible for them to participate in agriculture), instituting the death penalty for Jewish proselytization, and forbidding Jews to hold office or intermarry (despite their still technically being equal Roman citizens).

In the year 613, a far harsher decree was enacted by King Sisebut- the forced conversion of all Jews in Visigoth Spain on pain of 100 lashes, confiscation of property, and ultimately banishment. This was, technically speaking opposed by many Christian clerics such as the prominent leader Isidore of Seville (on the grounds that a forced conversion was no conversion and that secret reversion was likely, not out of any sympathy to the Jews- Isidore had been at the forefront of other anti-Jewish efforts), yet it was allowed to stand by the Church, leading to an era of terror in which it's possible that, until the Moorish conquest, nearly 100,000 Jews were forcibly converted, with others managing to escape. Children were taken from their parents for conversion (though apparently in some cases, Christian neighbors would send their children to be baptized in place of the Jewish children, sparing them) and those who had converted were forbidden to revert to Judaism. This did not stop many of them from retaining crypto-Judaism which was punished by the Church, an ominous foreshadow of what would occur in Spain 800 years later.

The next hundred years were a dizzying cycle in which different kings (who generally assumed the crown via conquest rather than birthright) implemented vastly different policies against Jews depending on what was deemed politically expedient. Some kings were relatively lax to the Jews and even turned to them as allies, but these kings were often deposed or assassinated, with their successors then not just reverting to the old anti-Jewish decrees but often implementing new ones. Many enforced massive oaths of allegiance to Christianity in Jewish converts such as forcing them to have no further contact with Jews, call Judaism "the vomit of [their] former error," and forbidding them from traveling without permission from their parish priests. King Erwig added a law that all transactions between Jews and Christians had to begin with all parties reciting the Lords' Prayer and eating pork; he asked the Twelfth Council of Toledo (whose members believed that the Jews had to be cut off like a cancer) to help him eradicate the "leprosy of Jewish corruption." During the subsequent regime of King Egica in 694, following an allegation of Jewish planned rebellion, the council enacted new laws declaring Jews to be slaves, requiring that their property be confiscated and their young children turned over to Christians to be raised in Christianity. (The evidence is unclear, however, as to whether the decree ever took effect.)

After all this, it should be pretty clear why, once the Moors conquered Spain and restored religious and personal freedom to Jews, they eagerly aligned themselves with them. However, while the assertion had been made by the Visigoths that the Jews were in league with the encroaching Moors pre-conquest, and while Christian sources for centuries after indicated that Jews were involved in a conspiracy with the Moors, after the fact this has largely been attributed to anti-Jewish prejudices and legends. In the 19th and early 20th centuries, the myth was held up as fact by such Jewish historians as Heinrich Graetz and even Salo Baron, who generally assumed that Jewish as well as Muslim Berbers conquered Spain, with the assistance of both Jewish refugees from Spain and, possibly, their compatriots still in Spain under Visigoth rule. Many eighteenth, nineteenth, and even twentieth-century non-Jewish scholars made even stronger claims, in many cases assuming that Jews were the real drivers of the conquest.

The fact remains, though, that there is no evidence from surviving Muslim sources that any Jews were involved in the conquest, and there is no evidence from either side that Jews in Visigothic Spain had any role either (in fact, there is little reason to see why they would, as past Muslim conquests in other parts of the world had led to forced conversion to Islam, and therefore the possibility of more of the same). The Moors had been independently planning to conquer Spain for decades by the time that they actually did, and while they certainly took advantage of a conspiracy to do so (a succession battle led the heirs of the former Visigoth King Witiza to ally with the Julian, the governor of Ceuta who had a quarrel with the current Visigoth King Roderick, to allow a small force of Muslim invaders in, soon followed by a much larger invading force led by Tariq ibn Ziyad), there is no evidence that any parties were Jewish (despite at least one historian claiming that Julian was Jewish, and one claiming that Tariq was!). We do see records from Muslim conquerors in which they assert that they left various conquered cities in the hands of the Jews, but this is attributed either to the Christians having previously fled the cities or to their knowledge of the Jews' mistreatment and therefore potential willingness to align themselves with a benevolent conquering force, NOT to prior contact and coordination.

I don’t know if it helps but I wrote this answer about the invasion and focused specifically on the impact of Basque/Frankish and Jewish elements in the early stages of the conflict.

It includes an excellent source which focuses directly upon Jewish involvement in the events and why they did so.

Hope that helps.