You must be Level 70 and above to do the party quest, which is also known as the Orbis PQ.
For a complete video guide of this PQ, click the Play button above (thanks to AionJC).
Enter the map through the dimensional portal (shown above) and form a party of 3 or greater. The recommended levels for this quest are 70-119.
Your party leader must talk to the NPC Wonky the Fairy to begin the party quest. You will have 20 minutes to complete the party quest.