If people love your game/site/product/etc., they'll be compelled to talk about it, brag to their friends about their involvement with it, share ideas about it, complain about it... in short: they'll feel a natural compulsion to get involved.
While working on an idea for a product, make sure that the community elements are intrinsic* to your product. Your product must be comprised of many things, one of which must be some sort of mechanism for community involvement.
For example, if you're building a game, insure that one or some of the core mechanics implemented are dependent on community involvement. For a non-game, insure that there's some necessary community involvement from the earliest customer contact.
From the start, make certain that community is one of your behavioristic design components**.
The alternative is to build something, launch, and subsequently "bolt-on" a community, usually in the form of a bulletin board/forum. What happens in this case, unfortunately, is that (usually) by the time you get around to it there's already one out there created by your fanatic customers, and you now have to either co-opt theirs or build your own and compete with it.***
*adjective: belonging naturally; essential
** And please, don't be cynical with your behaviorist design. We're not mice, and you're not building a Skinner Box.
*** When Paul and I built "Just Three Words", two separate forums popped-up within a week of each other, very quickly after launch. We'd missed the boat on creating our own, and so became members of a customer-operated community.