Reach out to your potential audience via email using AB testing
Digital using tools like SurveyMonkey or in person/on the street surveys to ask very directly what they need and whether they would use tools you build.
Related to surveys, look at existing solutions that overlap your own. Why are they successful? What are they lacking? Is your project addressing those?
Before you answer this question, you need to know what longterm success for your project looks like. Is it facebook likes? link shares? form entries? potition signatures? physical activities?
Next, create checklists or step-by-step instructionals for leaders or members so they know what they can do to ensure further action happens.
Ways to report back their successes or failures.
Look for overlapping community and do targetted outreach. Google ads, on the street outreach, mailing lists, exit poll. Google has grants for AdWords
Be creative, What is the google autofill for some subjects related to your field? Try Google Trends search
Target the leaders of existing communities and ask them to share your work.
Many websites ‘stick’ a lot better when a user is walked through the first stages of engagement. This can be done through clever HTML or just by having step by step videos places carefully to guide the user through the early stages of your actions.
Forums allow your users to create questions or generated conversations that promote actions. They also allow people to define new motivations or calls to action.
Give you users tools but also let them know that you are there to help them in the process of doing good things. Ways to promote their actions. Highlight their conversations (from twitter, facebook, forum). See Nir Eyal