The very first thing to do is to make volunteers know what help the community needs and how they can contribute.
It is critical that your platform has easy-to-use tools that allow one to find the work the community needs and track its progress. One approach is to have a list of work needed currently. The list can be generated automatically or maintained by someone.
This is a fragment of a draft of the book “Lessons Learned While Working On Stack Overflow”. Read the full book on kindle or the paperback version.