Albert Barnes Commentary Esther 8:17

"And in every province, and in every city, whithersoever the king`s commandment and his decree came, the Jews had gladness and joy, a feast and a good day. And many from among the peoples of the land became Jews; for the fear of the Jews was fallen upon them." — Esther 8:17 (ASV)

Became Jews - Joined the nation as proselytes, thus casting their lot with them.