A mobile user device for use in a peer-to-peer data sharing group comprising at least two mobile user devices inclusive of the mobile user device, each mobile user device of the data sharing group configured to store data shared by mobile user devices of the data sharing group on the respective mobile user device and to maintain a list of the mobile user devices in the data sharing group, wherein traffic from any one mobile user device is directed through at least a relay or router configured to direct traffic to the mobile user device of respective members of the group, and wherein the relay or router is configured to accept from the mobile user device of a sending group member a single message containing the addresses of multiple receiving group members, and to re-distribute the message independently to the mobile user device of each of the multiple receiving group members, the mobile user device comprising: an output interface configured to transmit data to at least one other mobile user device of the data sharing group, the output interface comprising a port connectable to a wireless network; a user interface comprising a touch display screen configured to receive an instruction to enable location data sharing for a limited time period after which location sharing is to be disabled; a processor coupled to the output interface and to the touch display screen; a battery to power active elements of the mobile user device; a location sharing module configured to, during the limited time period, provide location data of the mobile user device to the output interface to be transmitted to the at least one other mobile user device of the data sharing group; and a location determination system from which the location sharing module receives the location data; wherein the location sharing module is further configured to instruct the location determination system to turn off after the limited time period. each mobile user device of the data sharing group configured to store data shared by mobile user devices of the data sharing group on the respective mobile user device and to maintain a list of the mobile user devices in the data sharing group, wherein traffic from any one mobile user device is directed through at least a relay or router configured to direct traffic to the mobile user device of respective members of the group, and wherein the relay or router is configured to accept from the mobile user device of a sending group member a single message containing the addresses of multiple receiving group members, and to re-distribute the message independently to the mobile user device of each of the multiple receiving group members, the mobile user device comprising: an output interface configured to transmit data to at least one other mobile user device of the data sharing group, the output interface comprising a port connectable to a wireless network; a user interface comprising a touch display screen configured to receive an instruction to enable location data sharing for a limited time period after which location sharing is to be disabled; a processor coupled to the output interface and to the touch display screen; a battery to power active elements of the mobile user device; a location sharing module configured to, during the limited time period, provide location data of the mobile user device to the output interface to be transmitted to the at least one other mobile user device of the data sharing group; and a location determination system from which the location sharing module receives the location data; wherein the location sharing module is further configured to instruct the location determination system to turn off after the limited time period. an output interface configured to transmit data to at least one other mobile user device of the data sharing group, the output interface comprising a port connectable to a wireless network; a user interface comprising a touch display screen configured to receive an instruction to enable location data sharing for a limited time period after which location sharing is to be disabled; a processor coupled to the output interface and to the touch display screen; a battery to power active elements of the mobile user device; a location sharing module configured to, during the limited time period, provide location data of the mobile user device to the output interface to be transmitted to the at least one other mobile user device of the data sharing group; and a location determination system from which the location sharing module receives the location data; wherein the location sharing module is further configured to instruct the location determination system to turn off after the limited time period.