Did you guys register? you can't participate if your leader and seniors forget to register.
Show the registration notification mail plz

I was going to say that, but I looked at the screenshots carefully and...
708, more than enough, even if it fluctuate a bit....

When a group is registered, every member receive a mail. "Pretty sure" means nothing, dig up the mail (the mail would be sent out between Monday, the earliest possible registration day, and Wednesday, the deadline, so shouldn't be too hard to find)
If you deleted your mail, ask your group mates.
if no one have the mail, your leader forgot.
My leader have forgotten and we ended up not in war once, it's not impossible.

You can't say that.
objectively, you can only say that "he says" he didn't forget.
The problem is that, if he is telling the truth, he didn't notice the missing mail when he registered, which is a bug in and of itself. If he did and reported that before the registration deadline, then you guys would have a perfectly good case with proper evidence. But now, there is no way to prove it either way, so I'm afraid nothing will come of it.
That said, based on the fact that two different group are impacted in a short time spam (assuming the other group registered last time), if there truly is a bug, it's likely to repeat itself. Pay attention to what happens when you guys try to register and catch it in the act, then you'd have a strong case and can come back to this and request compensation. Spread the word to the other groups to have them look out for the bug too, as it doesn't matter which group validate the bug.
But until done, I seriously doubt anything would come of "he says"