The basic NPC membership requirements:
NPC Associate Membership -- The sorority must have 12 collegiate chapters and an established national character for 9 years. National character is like a National Council/Board that oversees the chapters.
NPC Active Membership -- The sorority must be NPC associate member for 4 years, with an established national character for 13 years; and have a minimum of 14 collegiate chapters, with the 14th chapter being at least 2 years old.
Some previous threads on joining NPC:
http://www.greekchat.com/gcforums/sh...ad.php?t=78789
http://www.greekchat.com/gcforums/sh...t=69836&page=2
http://www.greekchat.com/gcforums/sh...ad.php?t=26038
Phi Sigma Rho considered the option of NPC associate membership several years ago, but decided not to apply due to NPC views on extension. I believe my sorority is looking into starting another women's conference with a more similar view on extension like the NIC.