Ok so I think I know the awnser to your question...
I believe that NPC allows you to be in other GLO's but they can't be social. Like I could be part of Alpha Phi, and I could also be part of Alpha Phi Omega (a national service fraternity). When it comes to locals I think that the same rules apply because they are not national. I hope that awnsers your question, if I am wrong ppl let me know.