Growing your community depends on the type of community and members you have, your missions, and your vision. For some people growing community means an increasing number of their members, for some, is improving the quality of the content generated. So, you first need to know if you care about growing the quantity or quality.
For me, it's been several things. The most significant channel I used to grow my community was to increase the number of active members in a month. I was trying to show it to my members that their sharing knowledge is very valuable. So I took the scoring and badging system very seriously and emphasized the quality of activity. I gave my active users some of the admin accessibilities, and surprisingly that worked very well. Members wanted to show off and say "Hey, I am different". They were competing to get more points, better badges, and more accessibilities.
The other channel was getting backlinks from websites with a significant number of visits. When a reliable website talks about you as a reference, more people would trust you too.
Never forget social media engagement. Social media channels such as Facebook, Twitter, and LinkedIn has the power to help you reach a wider audience around the world. Don't be afraid about creating unique content in your social media; if you see something helpful on the internet, share it and ask your audiences to share the opinions around that topic.