Wasn't this made eventually so that you all teleport to the same instance while in a party? It was a common complaint, so they fixed it.

The only time when this doesn't work is when the party was made after teleporting to the zone, or if the instance is full. And there's nothing to be done about that, exceeding the limit just to keep a group together would defeat the purpose of instancing the zone in the first place.

You can change instances at any Aetheryte in the zone btw. As for telling which instance you're currently on, just type < pos > without spaces.