I don't know why everyone's saying Janis used Cady for revenge.

Janis didn't really expect Cady to bond so well with the girls. Notice she started slowing drifting away from Janis and Damien to the point where she didn't even invite them to that party or go to Janis art show.

Cady only began hanging with them b/c of Janis's sceem. It wasn't until Cady got carried away by acting like Regina that Janis started to get mad at her.