E2-BAG/1/7 is a UNSC Marine Corps unit.
Background
This unit and it's members were stationed on Earth in 2552. They probably fought in the First and Second Battle of Earth.
It is possible the E stands for the company, Echo. The 2 may be second platoon of E Co, or second battalion, which the company is part of. BAG could stand for Brigade (or Battalion) Assault Group, which would be the infantry element of a reinforced Marine regiment or battalion, or Marine expeditionary unit. The 1/7 could either be 1st Battalion, 7th Marine Regiment, or 1st Regiment, 7th Division. This is simply conjecture and should not be taken as canon.
E2-BAG/1/7 was interviewed by Dr. Robert McLees on multiple UNSC and Covenant vehicles and firearms that will appear in Halo 3. Some of these interviews were posted on Bungie.net's front page.