1. A method comprising: receiving, at a trusted service, a first plurality of tokens and corresponding timestamps from a first device, where the first plurality of tokens do not identify a particular device or its user or owner and where the first plurality of tokens were received by the first device from a first plurality of devices; receiving, at the trusted service, a second plurality of tokens and corresponding timestamps from a second device, where the second plurality of tokens do not identify a particular device or its user or owner and where the second plurality of tokens were received from a second plurality of devices; comparing, at the trusted service, the first plurality of tokens and timestamps to the second plurality of tokens and timestamps; based on the comparison, determining, at the trusted service, that the first device and the second device have collected one or more matching tokens at a time, or in a time frame, determined by the timestamps; identifying, at the trusted service, users associated with the first device and the second device; and creating, at the trusted service, a group for the identified users.