You can trade without a fee with everyone in your fellowship, whether you have discovered them on the map or not.
You can trade without a fee with everyone you have discovered on your map.
You can trade with some others you have not discovered yet, but only with a 50% fee added, so it doesn't really make for worthwhile trades most of the time.
Only way is to scout and discover more players, though in the fairychapter there is a Wonder that will reduce the fee a bit.