It has been along time since "build it and they will come" has been an effective strategy for ensuring participation. It might work well for baseball diamonds, soccer fields, or hockey rinks, but not for public participation. A comprehensive outreach strategy is essential to encourage participation by reaching out to those you need to hear from. For government that means getting beyond the "usual suspects" by considering who you would like to hear from and finding out a means of coordinating with them.
Some key elements of the strategy should include:
Target audience analysis - who is out there that should participate?
Messaging - why should they participate? what is the rationale?
Promotion and advertising - how to reach them?
Viral - how to get participants to invite other participants?
This is a very quick overview of the key elements to creating an outreach strategy that will build the initial participation.