Sue did participate in some of the initial bullying of Carrie, particularly IIRC the infamous period scene in the showers. But Sue, unlike most of the other girls, felt remorseful. She felt bad about bullying Carrie and wanted to do something nice for her. So she asked her boyfriend, who was handsome and popular, and who Carrie had a crush on, to take Carrie to the prom as a way to show the girl a good time, and perhaps help integrate her more into the social scene of the school. Sue and Tommy aren't involved in the plan by other students, lead by Chris, to humiliate Carrie. When the blood drops on Carrie, you can see that Tommy is genuinely pissed.