In group activities, members often could hurt others' feelings by using smartphones more than expected due to the lack of awareness on smartphone usage. Other members can remind them of their usage by asking not to use their smartphones, but our survey results show that explicit asking could bring about negative experiences in a group. To overcome this, we designed LockDoll, a tangible artifact that provides ambient feedback of smartphone usage to help members manage their usage. By following an ambient design pattern of symbolic sculptural display, we expect LockDoll to softly and yet, effectively intervene with smartphone usage in the group context. In this paper, we present the preliminary working prototype and discuss further design issues through a pilot study.