- Advertise it on different forums/youtube.
- Tell your friends in game about it and get them to tell their friends etc.
- Get someone to promote you, for example for world of warcraft there is Athene/Swifty/Hobbs which sometimes promote people.
- You could make a video to put on Youtube and if someone who has a lot of subscribers likes it, that's bound to bring in more traffic/users.
Nothing much more I can think of really, it's all about spreading the word.