1- Open forum on your site (as juhaw said)
2- Open blog to every register member (send email to member when he will get a message)
3- Open communities on your site, allow members add their photos and send other members messages.
4- Use Advertisment or buy visitors from www.easytraffic.biz
All the advice on this thread is great. I also recommend:
1. Create a Twitter account and be an active and engaged user. Dont just post your site's url. Contribute and retweet and comment on others tweets.
2. Join related meetups - not only you meet others but it gives an opportunity for learning about other businesses and potential business partners.
Without overmystifying it, you need to
- your priorities
- your budget
- the amount of time you want to invest in
- your ability and skills in writing
if I were you and would have enough time I'd write my own articles putting my knowledge, my personality and a bit of my humor into them. Then if I get some money back I'd outsource it to an adequate person.