Your first step should be to join online communities and message boards that already exist for the types of interests you are trying to promote. If, and I can't stress this enough, they are okay with you advertising your community on their boards then you could go from there.
Just be sure that you are offering the users something unique that draws them to your group. Most people will not leave a community they are already in to be a part of another that is newer and exactly the same (but probably not as good, at least in their mind) as the one they are already in.
Use descriptive key words for your group on Steam to help people find it in searches.
Remain active in your own community and stay engaged. People like to see how things are changing and being improved.
The above statement is proved over and over when I see guys from my community from over 4-5 years ago checking in time and time again to see if we've re-released any servers or changed anything.
I've been running a community for the better part of a decade, and those are just a few of the things I've learned about how to make it work.
If you have any more questions, I would be happy to help you out.
