We just got back last week, and HP area was ridiculously crowded. We kept loosing people in the crowds. There are just so many different sides and entrances and exits and everything is crammed so close together, it is very hard to pick people out of the crowd even when they are nearby. The best place we found to meet if lost was in front of the snowman. It's an easy place to find, out in the open, so it's easier to find your party there. Kids can even sit up on the wall in front of it to get a better view over the crowds if needed. After some trial and error, we discovered this location worked best for us. (Also group of 6 with kids ages 11-13.)
The gift shop for Forbidden Journey is packed with two different exits, we had issues meeting there. If you do meet there, I'd try over by the photos.
The dragon coasters have a separate entrance and exit, so make sure to specify which you will meet by. The exit is also very long with many different places to wait and two ways out. You can go either behind or beside Olivanders, so make sure if you meet at the exit, you meet around the corner before the split. There's a little viewing spot off to the side in the exit queue where you can watch the coasters go by. I'd try there; although, if it's too crowded it might not work well with a large group.
I think the key is to just be really specific. At first we weren't specific enough, and ended up spending a good amount of time looking for exactly where at the exit or entrance the kids were waiting. If I did it again, I'd have the whole party go first to the exit, point out a specific bench, or tree, or landmark and then designate that as the spot to meet at. Then I'd have everyone go to the entrance to ride. This wasn't necessary in the rest of the park because the crowds were much thinner, so a quick scan usually turned up wherever the kids had gotten off to, but in the cramped crowded HP world it was a whole different ball game.