Welcome new people, and after the first ones get to know each other, continue to encourage oldtimers to welcome newcomers. Eventually, the community takes over the public welcoming function. People's first reactions are most important. Praise them by name. Read their profiles and point them to information that you think will be personally relevant to them.
Names have power.
Put your newcomer up on a pedestal by name, for doing something that adds to the community, and that newcomer will amplify that behavior forever after. Communicate via email with both promising newcomers and troublemakers. Don't stir people up in private, but always respond to email queries about social problems by encouraging people to act on their principles publicly. Encourage people to talk among themselves. Check your participation files and learn who your regulars are.