The explanation was given and is straightforward. C leader just discovered all VIPs by collecting all the phones from everyone. As for the group Horikita was in, he found out who the VIP was by reading Horikita's expressions. With knowledge of who the VIPs are, he can easily make sure C wins the game.
It's interesting that B-Blonde already knows it isn't Horikita but Ayanokouji himself who is the brains behind Class D. She seems way more competent that people give her credit for.