In this work, we present a novel human-robot interaction (HRI) method to detect and engage passive subjects in multiparty conversations using a humanoid robot. Voice activity detection and speaker localization are combined with facial recognition to detect and identify non-participating subjects. Once a non-participating individual is identified, the robot addresses the subject with a fact related to the topic of the conversation, with the goal of promoting the subject to join the conversation. To prompt sentences related to the topic of the conversation, automatic speech recognition and natural language processing techniques are employed. Preliminary experiments demonstrate that the method successfully identifies and engages passive subjects in a conversation.