We envision wearable social-behavioral assistants which measure the nonverbal behavior of their users during social interaction. Research in psychology has linked posture mirroring, a key element of nonverbal behavior, to rapport and empathy and has been found to support communication. In this paper, we present a method to measure posture mirroring in social interaction with body-worn motion sensors. Our method is based on the detection of basic posture classes and the comparison of displayed postures across group members. We apply our method in a group discussion scenario involving 42 groups consisting of three subjects each in which group leaders express different leaderships styles. Our results show that we can measure differences in posture mirroring across groups of different leadership styles.