Thanks.
I offered the first 15 sites free on a first-come, first-serve basis, which was while I was in beta yet. In a few cases, a few people had come to my own guild's site asking where it was hosted, and I redirected them to the hosting site. Otherwise, I've pretty much always just done Adwords, Yahoo, and MSN ads.
There wasn't a whole lot of convincing. I offered a 30-day money back guarantee and people just started signing up and paying even before I was advertising - those first few I have no idea how they found the site (I even had someone sign up and pay even though my Auth.net account was still in test mode so I just let them have the site free). Aside from that initial 15 guilds, I've never offered a free version. Every guild I've hosted has paid for it (with the exception of friends of mine that were starting their own guilds, of course).
I had (have) a "Featured Guilds" section which showed off what other guilds had done with the system, an online demo for people to tinker with the system before signing up, and offered more features than any other guild hosting site (or any open source package offered as far as I can tell). The only major guild hosting site out there that did this was stagnating from lack of competition.
That's what was so surprising about it...I wasn't expecting the degree of growth I had gotten at all.
It made a lot of promise to make easy what was otherwise a tedious thing.
Looking back, it just seems like it was "right place at the right time", and everything just...sort of...happened.
But like I said, the platform was in development for a year and a half before it launched, since it was my own guild's site, and I was completely serious that I wanted my own guild's site to be one of the best out there feature-wise.