There are unlimited pools of potential communities who just need a clear visible sign to rally around.
You need to create a sign which a) Fits perfectly with what people care about b) Solicits a social response and c) You can build upon into a cause/mission/common-interest group.
A newspaper article can be a sign. People read and rally around it. They discuss the story in the comments and a community forms from it.
A tweet can be a sign. People see the tweet, reply and engage with it, a community rallies (see Motrin scandal and #amazonfail).
A friend lives in a building with 8 other tenants. They didn’t speak to each other until, last week, he put up a sign on the communal door asking them to sign if they wanted to hire a gardener . It started conversations between a group of people who never spoke before.
A sign is quick, simple, viral, cheap. Instead of developing bespoke community websites or developing extravagant strategies, focus on putting up the strongest possible sign for people to rally around.