There is some difficulty in understanding how Zadok and Abiathar came to be both “priests” at this time, and in what relation they stood to one another. The best explanation seems to be that Abiathar was the real high priest, and officiated at the sanctuary containing the ark of the covenant in Zion, while Zadok performed the offices of chief priest at the tabernacle of Witness at Gibeon 1 Chronicles 16:39.
For Benaiah, see 2 Samuel 8:18; 2 Samuel 20:23; 2 Samuel 23:20-21. For Nathan, see 2Sa 7:2-3, 2 Samuel 7:17; 2 Samuel 12:1-15, 2 Samuel 12:25. As privy to all David’s plans 1 Kings 1:24, he had no doubt fully approved the order of succession which the king was known to intend.
Shimei and Rei - Shimei and Rei are perhaps David’s two brothers, Shimma and Raddai 1 Chronicles 2:13-14.
Mighty men - Probably the company of 600, originally formed during David’s early wanderings 1 Samuel 25:13; 1 Samuel 27:2, and afterward maintained as the most essential element of his standing army.