Hi Kelly. While I’m not a group cycling instructor, I do work a lot with groups. I would say, having a lot of enthusiasm as the instructor, smile a lot, motivate the group by raising the “group energy” (this can be by raising the level of your voice, having motivating music etc.), and model the behavior you want to see in your class participants (i.e. if I want my group to have fun, I MUST show that I’m having FUN!!!).
I hope that this helps.