Albert Barnes Commentary 1 Kings 1:8

"But Zadok the priest, and Benaiah the son of Jehoiada, and Nathan the prophet, and Shimei, and Rei, and the mighty men that belonged to David, were not with Adonijah." — 1 Kings 1:8 (ASV)

There is some difficulty in understanding how Zadok and Abiathar could both be “priests” at this time and what their relationship was to one another. The best explanation seems to be that Abiathar was the actual high priest, officiating at the sanctuary in Zion that contained the ark of the covenant, while Zadok performed the duties of chief priest at the tabernacle of the testimony at Gibeon (1 Chronicles 16:39).

For more on Benaiah, see 2 Samuel 8:18; 2 Samuel 20:23; and 2 Samuel 23:20–21. For Nathan, see 2 Samuel 7:2–3, 17; and 12:1-15, 25. Being aware of all David’s plans (1 Kings 1:24), he had no doubt fully approved the order of succession that the king was known to have intended.

Shimei and Rei – Shimei and Rei are perhaps David’s two brothers, Shimma and Raddai (1 Chronicles 2:13–14).

Mighty men – This was probably the company of 600 men, originally formed during David’s early wanderings (1 Samuel 25:13; 1 Samuel 27:2), and later maintained as the most essential element of his standing army.