I just kinda stumbled into it, actually!
I went along to my first MelbJS way back in 2011, I think it was. At that time it was being run by Anette Bergo from Thoughtworks, but she moved to Europe not long after that and needed to hand over the reigns. A few of us stepped up, including myself, but I ended up taking on most of the responsibilities. I created a new website, set us up on Lanyrd, started organising speakers. It really just snowballed from there, and I'm still going today.
I really love organising these events, helping to build a great little community, and JavaScript is such a rapidly evolving space that there's never a shortage of things to talk about.
You can see every one of our events on Lanyrd now, which goes all the way back to July 2012! http://lanyrd.com/series/melbjs