Albert Barnes Commentary Acts 9:28

"And he was with them going in and going out at Jerusalem," — Acts 9:28 (ASV)

And he was with them, and so on. That is, he was admitted to their friendship, and recognized as a Christian and an apostle. The time during which he then remained at Jerusalem was, however, only fifteen days (Galatians 1:18).

The phrase coming in and going out signifies that he continued to associate with them.