Not that I would not like to see a map ping function implemented, but your example does not really work.

If they were to be able to see your ping, they would have to be in your group, and if they are in your group, they will be able to see you on the map, so..

It would still be nice to be able to ping parts of the map to show people how to go someplace, for example into a cave/tunnel, it could be used to show where the entrance is.