First of all, Yes, Christian, Mark, Josh, Connie, and Simon were absolutely invited just to be sacrificed. They were dead the moment they set foot onto the commune grounds. That's why the elders let them have so much access. They were going to kill them anyway so it didn't matter what they heard or saw. Regarding Dani, the way I see it, she was just a target of opportunity. While talking to one of the Elders, they mention how incest is an issue. Getting Dani to come along would be a way to introduce fresh blood into the community. There is a time skip in between the death of Dani's family and the party. Pelle could have spent that time manipulating Christian into inviting her to come along.
I often wonder "How in the hell are they going to explain any of this?" after watching a lot of movies.