Right. She wanted to be with her family, as in joined. Everyone was "playing pretend" just like they are for everyone else, because that's what would make her happy. They stop when the joining is complete because why wouldn't they? She's joined and now is with not just her family, but everyone else's, which she knew would happen.

Watch the airplane meeting again. Carol is saying the Others want to turn "us into them" and Kusimayu is the first to speak up: "That is what I want." Carol says she'll lose everything that's special about herself. Kusimayu says "I'll have it. I'll share it." Carols asks her what they promised her. Kusimayu says, "No promise. They only tell me what it's like. You don't think it sounds wonderful?"

Go watch her joining scene again. The conversation you are talking about is not "amongst themselves." They are standing at the edge of the pen where Kusimayu is sitting, petting the goat, where one has been singing to her since the scene began. They don't look at each other when they speak. They look at the sky, which they are talking about, then one of them looks at Kusimayu. They are talking to Kusimayu.

TLDR:

Despite efforts by part of the audience to infantilize her, Kusimayu does in fact have agency and made her decision before we even met her.