There is a list on their website covering who can join but the basics are:
Be military (or Coast Guard), or retired receiving a check.
Sibling who is or parent who is one of the above
Through a room mate
Through a spouse
I think that covers the major ways, but like I said go to www.navyfederal.org and they have a list.
What's in my wallet: Cap1 Venture NFCU Sig Visa Delta Gold Amex BCE Citi Simplicity IHG World MC Amazon Visa CSP Starting Score: 5/2010 550-ish Current Score: 6/20/2013 TU 800 6/20/2013 EQ 770 6/20/2013 EX 775 Goal Score: 800 accross the board