Then the Spirit clothed Amasai, chief of the thirty, and he said, “We are yours, O David, and with you, O son of Jesse! Peace, peace to you, and peace to your helpers! For your God helps you.” Then David received them and made them officers of his troops.
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
And they helped David against the band of the rovers,.... The Amalekites, who, while he was gone with the Philistines, had seized on Ziklag, and burnt it, and carried the women captive, with what spoil they could make; these coming to David thus opportunely, assisted him in his pursuit and defeat of them: for they were all mighty men of valour, and were captains in...
Then the spirit came upon Amasai, [who was] chief of the captains, [and he said], Thine [are we], David, and on thy side, thou son of Jesse: peace, peace [be] unto thee, and peace [be] to thine helpers; for thy God helpeth thee. Then David received them, and made them captains of the band. (e) The spirit of boldness and courage moved him to speak thus.