I dunno if instancing is server or data center based, we went in to hard last night on Leviathan (Primal) with 7 of us and there was plenty of other groups in there. Just made a habit of shouting what island we were on and people would show up and help, I ended up just grabbing everything I could and we AoE'd shit down. Got 3 NM pops in a row at one point, was pretty glorious. I dunno if grinding Normal or Hard is more productive though, even pulling like 5 things at once it was taking forever to kill stuff. Our group was super disorganized (every time we'd finish a pull people would fly off, other groups not listening and trying to grind on other islands filled with awful mobs), but here was our final haul:
Might see if I can organize a big AoE zerg in Normal mode and see if the bigger/faster pulls make it better than normal. But you can definitely have a productive night in Hard mode without dragging multiple parties so long as your instance isn't completely dead.