1 Kings 1:7

Joab the son of Zeruiah Zeruiah was David's sister, and Joab the eldest of her three sons, and a man of much authority and influence during David's reign. He became captain of the host (as we should say, commander in chief) after the conquest of Jebus (1 Chronicles 11:6), and served David faithfully in the main until this time. But he held David's secret in the matter of Uriah, and accordingly defied him, when it suited his purpose. His greatest offence against the king was the slaughter of Absalom, whom he killed in defiance of David's order. (2 Samuel 18:14.)

Abiathar the priest This was the son of that Ahimelech who, with all his family except Abiathar, was put to death by Saul, when Doeg the Edomite informed him that the shewbread from the tabernacle at Nob had been given to David (1 Samuel 21, 22.). Abiathar then escaped to David and had remained with him through all his troubles until this time, when, it may be through jealousy of Zadok, he sided with Adonijah against Solomon.

they following Adonijah helpedhim] Lit. -They helped after Adonijah." Joab had his own thoughts about the orders which David might leave at his death, and Abiathar his jealousy, and both no doubt felt that by their conduct, if it were successful, they were making Adonijah for evermore their debtor.