The major open source development projects got it. Huge communities that have been successful include Apache, Linux, Wikipedia, Project Gutenberg and many, many others.
"Community exists because the members share a common purpose which can only be accomplished jointly."
.. 12 Principles of Collaboration
This is a good test for the necessity and viability of a community. What do members want to do that an individual can not? What's in it for me? How can I help? Different perspectives or skills may be important or helpful. If there is more work than an individual can do, others are enlisted and a community forms.
That in turn leads to a sense of identity - members and non-members, incentive for joining, who's who within the community, communication, reputation, and trust. Now that there are a bunch of folks participating, their efforts must be orderly, directed and respected. Which leads to governance (leadership, management duties) and groups within the community.
The appropriate environment (or platform) is instrumental to achieve their purpose. You know that you are there. The tools and resources are at hand. The environment promotes the exchange of value - ideas, money, contacts, links - whatever is of value within the community.
Finally, there must be an expression of soul or personality of the community, along with a history and record of the past. You need to "feel" it now, and keep reminders for later.
Can we facilitate a community into existence? Is there a minimum time to "cook" before the community dynamics develop? Is there a minimum number of active members for a "critical mass"? Do you really need a community or will some other, similar aggregation serve the purpose?