Kamikakushi (神隠し, lit. "hidden by Kami") means "spirited away". Kamikakushi is used to refer to the mysterious disappearance or death of a person that happens when an angered god takes a person away.
The wikipedia entries mention some works of fiction where people die or disappear in circumstances involving supernatural beings
The use of the word kamikakushi (literally "hidden by gods") within the Japanese title, and its associated folklore, reinforces this liminal passage: "Kamikakushi is a verdict of 'social death' in this world, and coming back to this world from Kamikakushi meant 'social resurrection.'"
The major themes of Spirited Away center on the protagonist Chihiro and her liminal journey through the realm of spirits.
In anthropology, liminality (from the Latin word līmen, meaning "a threshold") is the quality of ambiguity or disorientation that occurs in the middle stage of a rite of passage, when participants no longer hold their pre-ritual status but have not yet begun the transition to the status they will hold when the rite is complete. During a rite's liminal stage, participants "stand at the threshold" between their previous way of structuring their identity, time, or community, and a new way, which completing the rite establishes.
So, seeing as he says he was left behind by his friends, and that there are people bound and killed all around Senpou temple, it is likely they held rituals which were meant to take them closer to the divine beings so they can live a peaceful afterlife, but the Ashina came and killed them
And, our poor friend Kotaro is... Special
He probably ran away at the sight of the dead bodies, hence the crying and he thought that his friends "ascended" without him
And you send him to a place where he can join them again.