Be aware that every member of your group needs to have this mirror quest done in order to be able to progress further inside this instance.
If someone in your group is unsure; have them check their quest journal, or right click the mirror and see what option shows up. If they can select "Enter the Mirror" they can enter to the next floor. If they can select "Gaze into the Mirror" they need to get and/or finish the mirror's quest.
Once you have "entered the mirror" the only way to get back down is to evac or die. Clicking on the mirror you exit from will zone you out to The Shimmering Citadel, possibly locking you out from the instance.