Lay down on the right side of the ATC entrance, roll towards the wall. Here is a GIF showing how to do it:

http://giant.gfycat.com/GroundedWellmadeGalapagosalbatross.gif

This was done at NEAF.

Here is an image of one server I joined while I was glitched inside the building:

http://imgur.com/ZxdD20B

This was a group of random wall glitchers that all happened to join the same server, none of them were playing together.
This glitch is very prevalent as you can see. Most full servers I joined had at least one person glitched inside the walls, or groups like in the image, with dead players they had ambushed laying on the floor inside or outside the ATC.