And he conferred with Joab the son of Sarvia, and with Abiathar the priest, who furthered Adonias's side.

And he conferred with Joab the son of Zeruiah, and with Abiathar the priest; and they helped Adonijah and followed him.

And his words are with Joab son of Zeruiah, and with Abiathar the priest, and they help after Adonijah;

And he had talk with Joab, the son of Zeruiah, and with Abiathar the priest; and they were on his side and gave him their support.

And he conferred with Joab the son of Zeruiah, and with Abiathar the priest. And they followed Adonijah and helped him.

And he conferred with Joab, the son of Zeruiah, and with Abiathar, the priest, who gave assistance to the side of Adonijah.

Joab's defection on this occasion, after his faithful adherence to David during the troubles caused by Absalom 2 Sam. 18:2-17, may be accounted for by his fear that Solomon would be a "man of rest" 1-Chronicles 22:9 and by his preference for the character of Adonijah. He may also have thought that Adonijah, as the eldest son 1-Kings 1:5, had almost a right to succeed.
Abiathar's defection is still more surprising than Joab's. For his history, see 1-Samuel 22:20 note. Hereto, David and he had been the firmest of friends. It has been conjectured that he had grown jealous of Zadok, and feared being supplanted by him.

And he conferred with Joab - Joab well knew, if he made the new king, he would necessarily be continued in the command of the army, and so govern him.

And he conferred with Joab the son of Zeruiah, and with Abiathar the priest: and they following Adonijah (e) helped [him].
(e) They took his part and followed him.

And he conferred with Joab the son of Zeruiah, and with Abiathar the priest,.... About getting the kingdom into his hands: and they were very proper persons to consult with, who, if gained to his interest, might be of great service, the one being the general of the army, and so had a great interest in the soldiery, with whom he could make way for him, and defend him, and the other was the high priest, who might be thought to have a great share in the affections of the people, and whose office it was to anoint the king; and he might the rather apply to them, knowing them to be, on some accounts, discontented persons:
and they following Adonijah, helped him; they took on his side; Joab knowing David's hatred of him on account of his murder of Abner and Amasa, and especially for his slaying his son Absalom, and his insolent behaviour towards him, and perhaps he might fear, or had an him of what he had charged Solomon with concerning him; and Abiathar, who saw plainly that the priesthood in Eli's family was declining, and that Zadok was the favourite priest with David, and in all probability would be with Solomon; all which might influence these two persons to join Adonijah, and who, by so doing, greatly encouraged him, and many others to flock to him, which much helped and served his cause.

he conferred with Joab--The anxiety of Adonijah to secure the influence of a leader so bold, enterprising, and popular with the army was natural, and the accession of the hoary commander is easily accounted for from his recent grudge at the king (see on 2-Samuel 19:13).
and with Abiathar the priest--His influence was as great over the priests and Levites--a powerful body in the kingdom--as that of Joab over the troops. It might be that both of them thought the crown belonged to Adonijah by right of primogeniture, from his mature age and the general expectations of the people (1-Kings 2:15).

He conferred (for the expression, compare 2-Samuel 3:17) with Joab and Abiathar the priest, who supported him. אהרי עזר, to lend a helping hand to a person, i.e., to support him by either actually joining him or taking his part. Joab joined the pretender, because he had fallen out with David for a considerable time (cf. 1-Kings 2:5-6), and hoped to secure his influence with the new king if he helped him to obtain possession of the throne. But what induced Abiathar the high priest (see at 2-Samuel 8:17) to join in conspiracy with Adonijah, we do not know. Possibly jealousy of Zadok, and the fear that under Solomon he might be thrown still more into the shade. For although Zadok was only high priest at the tabernacle at Gibeon, he appears to have taken the lead; as we may infer from the fact that he is always mentioned before Abiathar (cf. 2-Samuel 8:17; 2-Samuel 20:25, and 2-Samuel 15:24.). For we cannot imagine that Joab and Abiathar had supported Adonijah as having right on his side (Thenius), for the simple reason that Joab did not trouble himself about right, and for his own part shrank from no crime, when he thought that he had lost favour with the king.

They helped - Either because they thought the right of the crown was his: or to secure and advance their own interest. It seems God left them to themselves, to correct them for former miscarriages, with a rod of their own making.