Dumbledore’s army was known as D.A. You were in your third year at Hogwarts, once the D.A. Meeting in hogs head was over they made whoever was participating write their name on parchment paper. Once you got done signing it you headed to class. The following night was the meeting, it was being hosted in the Room of requirement. Once you entered the double doors you saw Harry teaching others how to cast the expelliarmus charm. You quickly went over to a spot as you waited for Harry to explain.