This trophy is titled Bad Guy Boogie and it requires you to use Harley Quinn and her boombox to gather five citizens and make them dance.
Once you've progressed the story enough to control Harley in the hub, travel into Gotham and hold the Circle Button down to bring out her boombox. Once it begins playing music, wait on a street until five citizens have gathered around to begin dancing.
Once five are dancing around Harley, the trophy will unlock.