They can be found at the beginning of the level Assault On The Control Room if you jump all the way down to the bottom of the pit. You will die from such a high jump. Mods are not required to view this Easter Egg but they do make it easier to view. In the begining of the level when you are forced out of the pelican, hold X to reboard the pelican. The pelican will then fly down to the bottom of the pit. The pelican does crash into pillars which can force eject you from the vehicle and kill you. Doing this method on co-op increases your chances of reaching the bottom. It consists of three Grunts (two Grunt Minors and one Grunt Major), laying together with most of their geometry overlapping. It should be noted that the Grunt Minors are "Curlback" Grunts.
These are, in fact, the Grunts from the previous cutscene. Scripts in HEK (Halo Editing Kit) will show that the Grunts in the cutscene die and are teleported to the pit. This being said, it is not a genuine Easter Egg.
You can also view the Easter Egg by starting the level with two players, and having one player exit the Pelican while in free-fall. This is, however, difficult to pull off.