GroupTogether is a system that explores cross-device interaction using two sociological constructs. First, F-formations concern the distance and relative body orientation among multiple users, which indicate when and how people position themselves as a group. Second, micromobility describes how people orient and tilt devices towards one another to promote fine-grained sharing during co-present collaboration. We sense these constructs using: (a) a pair of overhead Kinect depth cameras to sense small groups of people, (b) low-power 8GHz band radio modules to establish the identity, presence, and coarse-grained relative locations of devices, and (c) accelerometers to detect tilting of slate devices. The resulting system supports fluid, minimally disruptive techniques for co-located collaboration by leveraging the proxemics of people as well as the proxemics of devices.