Beyond passive digital content sharing and displaying, we explored the potential use of mobile devices as a collaborative sensing system that can proactively monitor and mediate casual group conversations. Toward this goal, we developed Flower-Pop, a mobile system that monitors group conversations and visualizes interaction patterns by using a metaphorical expression with blossoms. Group conversation is mediated with spontaneous photo sharing and proactive conversation moderation that encourages inactive persons to share photos or selects a random person to speak. Our field study at a coffee shop and a department lounge showed that our system well supported animated participation in group conversations. Flower-Pop’s mediation guided smooth topic/speaker transitions and significantly encouraged less-active speakers to better engage in group conversation. Considering participants’ difficulties in disclosing proper photos at a timely moment, we discussed design challenges and opportunities that guide the design of mobile systems toward an active conversation facilitator.