There were certainly Jewish communities present in Medina at the time the muhajirun migrated there from Makkah.
At the beginning of the seventh century A.D., there were three Jewish tribes living in Medina (Yathrib). They were Banu Qainuka'a, Banu Nadhir and Banu Qurayza. All three tribes were rich and powerful, and also, they were more civilized than the Arabs. Whereas the Arabs were all farmers, the Jews were the entrepreneurs of industry, business and commerce in Arabia, and they controlled the economic life of Medina (Yathrib). The two Arab tribes – Aus and Khazraj – were debt-ridden to the Jews perennially.
Besides Medina, the strong centers of the Jews in Hijaz were Khyber, Fadak and Wadi-ul-Qura. The lands in these valleys were the most fertile in all Arabia, and their Jewish cultivators were the best farmers in the country.
The migration of Muhammad, the Prophet of Islam, from Makkah to Medina (then Yathrib), brought him into contact with the Jews for the first time. At the beginning they were friendly to him. He granted them the famous Charter of Medina, and they acknowledged him the ruler of their city, and agreed to abide by his decisions in all disputes. They also agreed to defend the city in the event of an invasion by an enemy.
But, unfortunately, this friendship did not last long.