In one embodiment, one or more servers providing an on-line service send a token to a first client computing device associated with a first user of the on-line service, the token being generated by the servers based at least in part on a user identifier of the first user; the serves receive the token from a second client computing device associated with a second user of the on-line service, the token being sent to the second client computing device from the first client computing device through a wireless connection; the servers verify the token based at least in part on the user identifier of the first user; the servers determine data accessible to the second user if the token is verified; the servers also send to the second client computing device at least some of the data accessible to the second user.