I sometimes volunteer with a group of students that does work maintaining woodland etc. They do different things in different places in the region, depending on who needs help with what. It's sometimes more effort to meet the group and then travel to the destination than it would be to travel to the destination directly. I was wondering if I could suggest that they use a GPS app on one of their smartphones so that people can make their way independently and then locate the group by GPS. This is only really practical if one app can communicate with all the various apps that volunteers might have on their different (Apple, Windows, Android, Other) phones. Is that possible or are they all proprietary and incompatible?