Other alternatives:
1) Get people to visit your site by adding content. Add meaningful stuff on your site beyond just the community, like articles, links, etc. Even if you're a community site, in the beginning you need an anchor to get people to reach your site and find it meaningful. Then if they like what they see, they may join the community at its infancy.
2) Get links from related sites. Ask, beg, give your puppy, etc.
3) Advertise in related sites or those your target audience frequents. (You didn't say you wanted to do this absolutely free.)
4) Partner with related sites, especially those with only content, and let them link to you so their visitors can discuss their articles, etc. Of course you link back and so on.