I'll echo another of the comments and say the brightness isn't so much the problem as much as it is the placement. I think the brightness actually adds to the effect you were going for. (I don't know if you're still looking for critiques on this though?? Just thought I'd try to help a little haha)
Hmm, I don't really think that the brightness itself is the problem here; overexposure can work very well if done properly. I think it's more the placement of the overexposed area: it would work a lot better if it was the background that was bright, instead of just the bit (grass?) in the lower part of the photo. The way it is now, it looks unbalanced. If you have some sort of photo-editing program that lets you edit parts of the picture, you could try brightening the wall (especially the upper and middle parts of it, to balance the bottom-right corner out).
As already mentioned by some, the brightness is a problem. The photo is generally overexposed and I would reduce the brightness to make it less overexposed. I might have composed it differently. I would suggest reading up on the rule of thirds and general composition articles . The pose of the cosplayers is quiet good and their faces are visible but not too visible. It kind of keeps some mystic to it!
Even though I would have quite a few things to say about composition here, you requested to focus on the issue of brightness, so I'll limit my comment to that.
I see just one problem with brightness:
Look at the right hand of the lady in pink. It totally blends in with her dress, which makes her arm look like it was cut short of the hand. You can solve this problem by reducing brightness in the dress and/or glove.
I would recommend making local adjustments in Photoshop Camera Raw (you can open JPGs in Camera Raw throught Bridge) with Adjustment Brush tool. Experiment with different brightness-altering methods.
Or you can do it directly in Photoshop: create a layer from the original file and add a layer mask that only reveals the white dress (in the layer mask, paint everything else with a black brush). Then, adjust the brightness of the dress separately of the rest of the image (which I think is OK brightness-wise).
Alternatively, you can also try using Recovery control in Photoshop Camera Raw to reveal folds in the dress and combine the two files using layers.
If any of these steps are unclear, please let me know.