A number of devices co-located at a geographic location can broadcast and receive tokens. Tokens can be exchanged using a communication link having limited communication range. Tokens that are received by a device can be stored locally on the device and/or transmitted to a trusted service operating remotely on a network. In some implementations, the tokens can be stored with corresponding timestamps to assist a trusted service in matching or otherwise correlating the tokens with other tokens provided by other devices. The trusted service can perform an analysis on the tokens and timestamps to identify devices that were co-located at the geographic location at or around a contact time which can be defined by the timestamps. A group can be created based on results of the analysis. Users can be identified as members of the group and invited to join the group.