We talk to prospective members before and after we launch a community.
Within a few weeks, we usually have 50+ long-interview transcripts, a clearly identified list of problems the group wants to solve (or goals they want to achieve), and a list of people who want to tackle the challenge.
Now we can launch a community around an issue members have told you they care about.
The issue has to be small enough that they can solve it (build momentum), but large enough to have an impact among the group (efficacy).
The issue must force them to interact (create social capital).
3 years ago, we worked on a community for teachers.
Every teacher says they are overwhelmed with work.
Dig deeper and they say they have too much marking, they are getting information from too many sources, they have to learn new technology too often.
We can't solve being overwhelmed. That's too big. But we can tackle that different departments each sending them information.
We solicited ideas (via the new community we sneakily created) and decided to have one simple digest of information from a single source. People that wanted the teachers to do something had to add it into next week's weekly checklist.
We then asked several of the most prominent teachers (again via the community) to begin contacting senior people to make this happen. It took a few weeks. It was a quick, easy, win.
It made the teachers feel good and realize they weren't helpless against the establishment. Next we focused on the technology problem. We arranged live webinars and feedback sessions (on the community) where teachers could suggest the best way to learn new technology.
The teachers were soon visiting the community regularly to get updates on the issue and suggest further ideas. At no point did we invite them to join a community about the topic.
The key is to personify the problem on something you can solve. Put the bigger issues aside. Focus on challenges that most members have. Then invite members to suggest ideas to resolve those challenges. Give the leaders more time and resources to tackle the issues themselves.