You don't need to form a party beforehand. the party forms as people sign up for the quest. Again, that is by design. I don't know why you can't understand that. It forces people to take turns instead of hogging the escort mission.
If you want content you can do with a pre-determined group there is plenty of that in the game, but the escort missions are not it. If they were, they would either be instanced, or multiple parties could take part simultaneously.