Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for tracking group members' proximity include associating a plurality of sensors with an application installed on each of a plurality of user devices and assigning each of the sensors to a corresponding one of the user devices. Assignments received on one of the user devices are wirelessly communicated to at least one other of the user devices, and proximity of each of the sensors to the corresponding user device is monitored. Status information is communicated between two or more of the user devices, and the status information indicates proximity of sensors to the corresponding user device and/or sensor assignments.