FileVault 2 (included with OS X Lion or later) can be used to encrypt your local startup disk. Use a local authorized user to unlock the disk. After the disk is unlocked, network users can log in as normal.
OS X: FileVault 2 and network users
A FileVault 2-encrypted disk is locked to network users who are listed as "enabled" in the FileVault tab of the Security & Privacy preferences pane.
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