Ad hoc formation and tracking of location-sharing groups

1. A computer-implemented method comprising:

transmitting, by a host device and to a mobile device, a location sharing request, wherein the host device is a mobile device to be included in a location-sharing group to be formed, the location sharing request identifying the location-sharing group and identifying the host device, the location sharing request including a parameter that specifies a duration in which a mobile device can be added to the location-sharing group;

receiving, by the host device and from the mobile device, a consent in response to the location sharing request;

forming, by the host device, the location-sharing group comprising the host device and the mobile device from which the consent was received; and

tracking a geographic location of the location-sharing group as locations of mobile devices included in the location-sharing group change.

2. The method of claim 1 , wherein receiving the consent in response to the location sharing request comprises detecting a movement of the mobile device according to a predetermined pattern.

3. The method of claim 1 , wherein tracking the geographic location of the location-sharing group as locations of mobile devices included in the location-sharing group change comprises:

tracking a geographic location of each mobile device included in the location-sharing group; and

providing the tracked geographic location of each mobile device included in the location-sharing group to at least one other mobile device included in the location-sharing group.

4. The method of claim 1 , further comprising:

transmitting, by the host device, the location sharing request to a plurality of mobile devices; and

receiving, by the host device, consents from the plurality of mobile devices in response to the location sharing request,

wherein forming the location-sharing group comprises including the plurality of mobile devices from which the consents were received in the location-sharing group.

5. The method of claim 1 , further comprising:

providing, by the host device and to a location-information server, identities of all the mobile devices included in the location-sharing group,

wherein tracking the geographic location of the location-sharing group as locations of the mobile devices included in the location-sharing group change is implemented by the location-information server.

6. The method of claim 1 , further comprising:

after forming the location-sharing group, transmitting a notification to the mobile device, the notification indicating that the location-sharing group has been formed.

7. The method of claim 6 , wherein the notification comprises respective consents from all mobile devices included in the location-sharing group.

8. The method of claim 1 , further comprising:

defining a geofence around the location-sharing group.

9. The method of claim 8 , wherein defining the geofence around the location-sharing group comprises:

determining, for each of mobile device in the location-sharing group, a respective coverage radius;

determining, for the location-sharing group, a boundary of a combined coverage area that encloses a current geographic location of each mobile device in the location-sharing group; and

determining the boundary of the combined coverage area as the geofence around the location-sharing group.

10. The method of claim 9 , wherein a coverage radius for a mobile device included in the location-sharing group is determined based on a connection type for a network connection established between the mobile device and another mobile device in the location-sharing group.

11. The method of claim 9 , wherein a coverage radius for a mobile device included in the location-sharing group comprises a user-specified distance.

12. A non-transitory computer-readable medium storing instructions executable by one or more processors to perform operations comprising:

transmitting, by a host device and to a mobile device, a location sharing request, wherein the host device is a mobile device to be included in a location-sharing group to be formed, the location sharing request identifying the location-sharing group and identifying the host device the location sharing request including a parameter that specifies a duration in which a mobile device can be added to the location-sharing group;

receiving, by the host device and from the mobile device, a consent in response to the location sharing request;

forming, by the host device, the location-sharing group comprising the host device and the mobile device from which the consent was received; and

providing, by the host device and to a location-information server, the location-sharing group, wherein the location-information server is configured to track the geographic location of the location-sharing group as locations of mobile devices included in the location-sharing group change.

13. The medium of claim 12 , wherein the operations further comprise:

transmitting, by the host device, the location sharing request to a plurality of mobile devices;

receiving, by the host device, consents from the plurality of mobile devices in response to the location sharing request;

including, by the host device, the plurality of mobile devices from which the consents were received in the location-sharing group; and

providing, by the host device and to the location-information server, identities of the plurality of mobile devices included in the location-sharing group.

14. The medium of claim 12 , wherein the operations further comprise:

providing, by the host device and to the location-information server, the consents received from the plurality of mobile devices.

15. A system comprising:

a host device comprising:

one or more processors; and

a host device computer-readable medium storing instructions executable by the one or more processors of the host device to perform operations comprising:

transmitting, by a host device and to each of a plurality of mobile devices, a location sharing request, wherein the host device is a mobile device, wherein the host device and the plurality of mobile devices are to be included in a location-sharing group to be formed, the location sharing request identifying the location-sharing group and identifying the host device and each of the plurality of mobile devices, the location sharing request including a parameter that specifies a duration in which a mobile device can be added to the location-sharing group;

receiving, by the host device and from each mobile device, a respective consent in response to the location sharing request;

forming, by the host device, the location-sharing group comprising the host device and the plurality of mobile devices; and

providing, by the host device and to a location-information server, the location-sharing group.

16. The system of claim 15 , further comprising the location-information server comprising:

one or more processors; and

a server computer-readable medium storing instructions executable by the one or more processors of the location-information server to perform operations comprising:

tracking the geographic location of the location-sharing group as locations of mobile devices included in the location-sharing group change.

17. The system of claim 16 , the server computer-readable storing further instructions executable by the one or more processors of the location-information server to perform operations comprising defining a geofence around the location-sharing group.

18. The system of claim 17 , wherein defining the geofence around the location-sharing group comprises:

determining, for each of mobile device in the location-sharing group, a respective coverage radius;

determining, for the location-sharing group, a boundary of a combined coverage area that encloses a current geographic location of each mobile device in the location-sharing group; and

determining the boundary of the combined coverage area as the geofence around the location-sharing group.