The easter egg in summit will give you a secret weapon.
This has been pronounced as Nuketown song...

Nobody has found it yet.
But I am SURE that I know what to do.

In the summit building in the big building there are golden boxes with buttons on the wall.
There are exactly 13 in the building. and 1 is at a place you can't get.
I also found the place where the weapon will come. It will come out of boxes just as in Zombies.

I'll show you guys a picture where the boxes are located. but first a picture how the boxes look like.
http://tinypic.com/r/2cmt18p/7
You see 2 golden boxes on the left and on the right.
Locations of 11 first and second floor:
http://tinypic.com/r/nmnjuq/7

Location of 12'th box:
http://tinypic.com/r/34yw681/7

Location of 13th box:
http://tinypic.com/r/2gxqwk9/7
This one is impossible to get to.

So there are exactly 12 of those boxes you can get to.
And the max party is 12 too..

The spot where the secret weapon will appear:

http://tinypic.com/r/11ccqbk/7
On the right you see like a fence, if you look through it you see a box that looks like a random item box.
Sorry for this bad picture.

I didnt test it out because I can't get 12 friends together  :P
But if it works please put it on uTube or something and send this guide with it.

We don't know how to activate these boxes, maybe you gotta press on the square and you will hear a click in the first seconds of the game?
Found by bm_wim and Siick_JunK

Thanks!