Quoting Commentary for I Kings 1:7

וַיִּהְי֣וּ דְבָרָ֔יו עִ֚ם יוֹאָ֣ב בֶּן־צְרוּיָ֔ה וְעִ֖ם אֶבְיָתָ֣ר הַכֹּהֵ֑ן וַֽיַּעְזְר֔וּ אַחֲרֵ֖י אֲדֹנִיָּֽה׃

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

Rashi

and Zadok as priest because Ebiathar had anointed Adoniah.
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