Silvermoon and the Sin'dorei of the Eastern Kingdoms joined the Horde at the invitation of the Forsaken under Lor'themar. Kael was on another planet, and took no part in this decision. Had the Warchief called the prince to Orgrimmar I doubt he would have answered. The alliance between Silvermoon and Undercity, and eventually inclusion into the Horde, was a necessity made precisely because Kael was preoccupied with serving Illidan.
Once the blood elves who had stayed behind sojourned to Outland, they did not find a monarch welcoming them and happy reuinion, they found Voren'thal and the Scryers who had abandoned their mad Prince for the Light of the Naaru (which, as his vision predicted, would ultimately come to Quel'Danas). Then the Horde Blood Elves fought the Illidari Blood Elves and helped depose Kael.
Kael, as I understand it, never personally joined the Horde. But that confusion is good, this isn't supposed to be cut and dry. It should be murky and encourage thought.