It is not clear if the men who argued with Stephen (Acts 6:9) or those who were upset by the accusations were the ones who seized him and brought him before the Sanhedrin. Either way, the diaspora Jews had a strong hand in his arrest. They also took the initiative later in Acts when it came to the arrest of Paul (Acts 21:28).1
12 And they stirred up the people and the elders and the scribes, and they came upon him and seized him and brought him before the council,