How Do I Use AirDrop In OS X Lion?
If you are nearby another computer that is using AirDrop, OS X Lion will automatically discover them. The computer should appear in the Finder with the user's contact photo.
You do not have to be connected to the same wireless network because AirDrop uses peer-to-peer WiFi between enabled computers.
When you're ready to share a file, just drag it to the person's contact photo - it's that easy. The recipient will have to accept the share requested before the transfer is initiated.
AirDrop creates a Firewall when establishing a peer-to-peer connection, so your transmissions will be safe from eavesdropping.