Ski Tracks doesn't have the ability to show you where other people are.
Vail resorts EpicMix does, but only to show you the most recent lift taken - it doesn't use GPS, just the RFID tag in your lift pass that the lift entrance scans as you pass. (I had it miss several lifts one day too.)
Those are the only two I'm familiar with.
From the website, I can't tell if Snowcru will show you where people are or not.
Crystal Ski Explorer looks like it does what you're looking for (and a lot more) but it's a British app so it only has a few US resorts in it. (I just downloaded it to check it out - it doesn't appear to be able to handle any resorts not listed. The listed ones in the US are : Aspen, Breck, Park City, Steamboat, Vail, Winter Park)
There are also a lot of apps that will just show you where people are, but aren't skiing specific. You might want to search for GPS tracking apps (or "track kids on phone", or something like that).