That's true.
Maybe you'd call it Get Together.
It'd have a flag for if you're out, and a map to show where your friends on it are, realtime. A bit like apple's find friends.
You could have an option to team up with people to show when you're in a group. This would mean you can see who else is going, and be able to get in touch with them too.
To be honest, I just looked at find friends just now and it's more or less all you need. The only thing would be to make a cross-platform version.