There are numerous good works that a congregation, denomination, or body of believers can perform in a community. Habitat for Humanity is one organization that involves volunteers from the community; often a local church will sponsor a Habitat home. Churches and individuals can help out in local soup kitchens, missions for the homeless and needy, and much more.
Perhaps the biggest way a church can aid in building up a community, though, is to practice what God teaches, living it out in a way that can be attractive to others.