According to its website, the first "Mission" of this American organization representing the interests of the US military-industrial-academic complex is to:

NDIA notes that its membership includes "the majority of the top 100 defense contractors."
http://www.ndia.org/Content/NavigationMenu/Membership_and_Chapters/Join_NDIA_Now!/Small_Business.htm

If one examines the listing of the "Top 100" published by Defense News, we find that NDIA could also boast that 16 of the top 20 military contractors in the world are within its ranks, including all of the top 8.
http://www.defensenews.com/static/features/top100/charts/rank_2007.php?c=FEA&s=T1C

Even having the so-called "Big Four" would be enough. Lockheed Martin, Boeing, Raytheon and General Dynamics—which have historically been the world's biggest, weapons producers—together win more than two thirds of all the Pentagons military contracts.

But the NDIA comprises more than just these big weapons makers. Its membership includes about 1400 corporate members (Corporate Member Portal) and "more than 47,000 individuals from the entire spectrum of the defense and national industrial base."
http://www.ndia.org/Content/NavigationMenu/Resources1/Mission_Statement.htm

The description that the NDIA provides of itself and its membership reflects its pivotal role as a unifying organization of the huge US military-industrial-academic complex.

"NDIA provides individuals from academia, government, the military services, small businesses, prime contractors, and the international community, the opportunity to network effectively with the government - industry team, keep abreast of the latest in technology developments, and address and influence issues as well as government policies critical to the health of the defense industry and the preservation of our national security."
http://www.ndia.org/Content/NavigationMenu/Resources1/Mission_Statement.htm