I believe the thing that says "New Member" under your name is sort of a rank. It changes once you reach different trophies and or a certain amount of posts/likes/etc.
Dan, planet minecraft is a totally different thing. It doesn't use buycraft or is a server. This system for forums, doesn't support buy craft. You will get "ranks" each time you post more threads.
It will later be added so the more post, likes, etc. you have the better rank you will have. I'm not sure what the bonuses of that ranks are yet or if there will even be any. Everything will be worked out soon so expect a lot of new features to come.
That would be pretty fun, though. I hope that idea would be added; it reminds me of the way they work things one of the game websites I used to play on when I was younger. Those were the days