greek question from a non-greek

[LuaBlanca]

so the gist of what i'm getting is
-the GLOs on campus "unite" and in essence do rush together...and the PNMs just pledge where they get a bid from??

now- if this is correct.......does that only stand for NPC sororities? (*meaning do the locals participate in that same formal rush as well?*) or do the local sororities/fraternities rush seperately - but at the same time...
or is it completely seperate?

**sorry if i'm being a pain but this is one major thing i was definately NOT understanding!**

Thanks Bunches!