Only one person in your group needs the key to open the gate in front of Shattered Halls. Also, a level 70 rogue with 350 lockpicking can pick the gate.

The quest to make the key starts in Shadowmoon Valley by killing Smith Gorlunk who is located on the Ata'mal Terrance of the Black Temple. He will drop a mold which starts the quest line to obtain the key.

Kill a Blacksmith orc called "Smith Gorlunk" outside The Black Temple, loot the ("this-begins-a-quest") item "Primed Key Mold" from him.

• Entry into the Citadel (Completes in Honor Hold for Alliance and Thrallmar for Horde)
• Grand Master Dumphry (Completes in Honor Hold for Alliance and Thrallmar for Horde)
• Dumphry's Request (4 Fel Iron Bar, 2 Arcane Dust, 4 Mote of Fire)
• Hotter then Hell (kill a Fel Reaver in Hellfire peninsula)

Quest reward for the quest "Hotter then Hell" is the Key to Shattered Halls.