To enter the Halls of Reflection and play the instance you will need to attune yourself by doing 2 previous prerequisite quests in the Forge of souls and Pit of Saron.
Or, you can open the new LFG, Select HoR specifically and wait for a random group. Upon entering chances are the event won't start, just have your party reset the instance a few times (Pref don't tell them you haven't done the prerequisite), and the event should start.
Completed this morning before I knew there was a prerequisite.
EDIT: You can no longer queue for HoR without prerequisite if you have entered ether FoS or PoS and havent finished quest. Only works if you are a virgin to the new dungeons. Just checked after completing FoS I can no longer queue for HoR
Or, you can open the new LFG, Select HoR specifically and wait for a random group. Upon entering chances are the event won't start, just have your party reset the instance a few times (Pref don't tell them you haven't done the prerequisite), and the event should start.
Completed this morning before I knew there was a prerequisite.
EDIT: You can no longer queue for HoR without prerequisite if you have entered ether FoS or PoS and havent finished quest. Only works if you are a virgin to the new dungeons. Just checked after completing FoS I can no longer queue for HoR