Well, I'm not entirely certain, so that's why I left a vague wording there.
When you start questing in Jade Serpent, there is no Sha invasion or anything. At that time, the Temple of the Jade Serpent is just a quest hub, with vendors, NPCs, etc. You can come and go as you like, and the zone is not instanced.
After the Sha invades the Temple, the only way to enter the dungeon is to enter the instance. When I first did Jade Forest on the beta, I clearly remember that the temple started showing up in my LFG interface from the moment I had completed the quest line where the Sha erupts in Jade Forest.