Now these were the men who came to David at Ziklag, while he was still banished from the presence of Saul son of Kish (they were among the mighty men who helped him in battle;
We have here an account of those that appeared and acted as David's friends, upon the death of Saul, to bring about the revolution. All the forces he had, while he was persecuted, was but 600 men, who served for his guards; but, when the time had come that he must begin to act offensively, Providence brought in more to his assistance.
Commenting on 1 Chronicles 12:1-22
Now these [are] they that came to David to Ziklag, while he yet kept himself close because of Saul the son of Kish: and they [were] among the mighty men, helpers of the war. (a) To take his part against Saul: who persecuted him.
Came to David to Ziklag - Achish, king of Gath, had given Ziklag to David, as a safe retreat from the wrath of Saul.