From what i remember the kind of default invite omes when you either reach legend or lvl 90.
I also know that how much money you spend has something to do with it. How much i don't know but i believe it factors in.
I think i made nation some where around lvl 68 and i was a Master just about to go TM. At the time i think i had spent about 50 bucks in total on the game. I was playing RGs everyday and placing and winning credits int them. So i also believe if you are entering tourneys and getting top 10 finishes that might have something to do with it as well, but that is speculation on my part.
Thats about as much as i know. Probably kind of like the tiering up system, a lot of people probably have a good idea of what it is but only WGT knows for sure