Hmmmm...you could say "Nyuukai shitekara no isshuunen omedetou gozaimasu" - meaning "Cogradulations for your one year since your joining"
....there's a couple more ways to say it.
o_O!?Hm? You're gonna have over 500posts!! You're gonna be an Elite member...not even a month after joining...wow.XD
...I've been here for a year and I've only got 860+ posts. XD