Well I know for a fact that EVERY NA LS on my server only parties within their LS's, save for a handful of people. and I mean a handful, 2 or 3 other NA players on my server play during Japan prime time as well. Almost every Japanese player on my server is fine with playing with random people. I rarely see party recruits or searches from NA players, only Japanese. Seemingly, when it comes to playing with other NA players, its probably best to get into a LS with people that go do leves and/or grinding together a lot.