If a programmer contacts you and asks to contribute to your project, how do you handle it?

• You don't know if this guy is any good. Perhaps he'll be more trouble than he's worth.
• He might be trying to attach his name to a successful project just for the kudos.
• He might be trying to take the project in a direction you don't really want, adding features you think aren't worth the extra complexity.
• Or, he might be a very useful contributor. You just don't know.

How do you handle such requests from people you don't know (On GitHub, specifically, if that makes any difference)? What's the etiquette here?