Possible meanings are 1) the Gentiles beat Sosthenes in the court in front of the judgment seat because he was the Jewish leader or 2) Sosthenes was a believer in Christ, so even though he was "the ruler of the synagogue," the Jews beat him in front of the court.
This may be an exaggeration to emphasize the strong feelings the people had. Alternate translation: "many people seized" or "many of them grabbed" (See: [[rc://en/ta/man/jit/figs-hyperbole]])