I know this question has come up before but I am too lazy to search.
I think that to join NPC, you must have at least 13 chapters and the newest must be at least 5 years old. (I think that's to keep from admitting groups who make a "big splash" and then don't hold up over time.)
I think that for some of the multicultural groups to join NPC they would have to make a LOT of changes to their rush and pledging programs. IMO, it's more likely a social/professional group like Ceres or Sigma Alpha Iota would join.
I'll let one of the lovely NPHC ladies on the board answer that part of your question.