I helped building producthunt.com and overclockers.at (and other less successful ones)
Here a few learnings:
1. the community already exists, you just create a communication platform for it
2. make it clear what the community is about [positioning/marketing]
3. make sure the communication/content is interesting [quality]
4. make sure there is enough engagement [perceived critical mass] (encourage people to post, post yourself a lot, fake accounts if needed, only create subforums once the main ones are noisy)
5. have a rhythm - some communities need daily good posts, some live of the weekly newsletter
Here a few learnings:
1. the community already exists, you just create a communication platform for it
2. make it clear what the community is about [positioning/marketing]
3. make sure the communication/content is interesting [quality]
4. make sure there is enough engagement [perceived critical mass] (encourage people to post, post yourself a lot, fake accounts if needed, only create subforums once the main ones are noisy)
5. have a rhythm - some communities need daily good posts, some live of the weekly newsletter