A.T. Robertson Commentary Acts 13:5

"And when they were at Salamis, they proclaimed the word of God in the synagogues of the Jews: and they had also John as their attendant." — Acts 13:5 (ASV)

Proclaimed (κατηγγελλον). Imperfect active of καταγγελλω, inchoative, began to proclaim. This was Paul's rule of procedure, "to the Jew first" (Romans 1:16; Acts 13:46; Acts 17:2; 18:4,19; Acts 19:8).

They had also (ειχον δε κα). Imperfect active, descriptive.

As their attendant (υπηρετην). Literally, "under-rower" (υπο, ηρετης) in the trireme. Probably here minister (χαζζαν) or assistant in the synagogue as in Lu 4:20. Cf. Matthew 5:25. It is not clear what John Mark did, though he was evidently selected by Barnabas as his cousin. He may have helped in the baptizing. There were probably others also in the company (verse 13). The "also" may mean that Mark did some preaching. Barnabas was probably the leader in the work in these Jewish synagogues.