A.T. Robertson Commentary Acts 4:15

"But when they had commanded them to go aside out of the council, they conferred among themselves," — Acts 4:15 (ASV)

They conferred among themselves (συνεβαλλον προς αλληλους). Imperfect active again. With Peter and John and the lame man outside, they began to compare (συν, βαλλω) notes and take stock of their predicament.