In the anime, it is shown that Kaguya regularly sends people to the Divine Tree for this "ritual" and those people somehow die.
So what exactly happens to them? I thought the Infinite Tsukuyomi was supposed to make people dream in their ideal world... Then how do people who go there for the ritual end up dead, whereas the ones who were first caught in it (King Tenji and others) live in their ideal worlds?
They live in their ideal worlds but while that's happening the tree is turning them into white Zetsus and I think somewhere in that process their brains turn to mush killing them. Thats where they got the army of Zetsus in the first place (by sacrificing people with the ritual turning them into Zetsus).
If I remember correctly, kaguya came to earth to build up an army of white zetsus in order to fight the villains who appear on Boruto the movie, (Momoshiki and Kinshiki) although it is never shown how the ritual could possibly take place. Lord Tenji and the others lived in their ideal world because Kaguya cast the infinite tsukuyomi upon them. And again when Madara casts it everybody lives in their ideal world. But during the backstory of Hagoromo and Hamura, it just shows a line of people heading towards the divine tree. The divine tree is basically the ten tails, so maybe it has a more crude way of performing the infinite tsukuyomi if Kaguya does not.
