An RFID reader

You can easily build an RFID reader using parts available from SparkFun electronics.

The carrier board connects to the BeagleBone through a USB cable, and will be seen by the operating system as a simple serial device.

RFID reader (image is CC BY-NC-SA 3.0)

You plug your RFID module into the board, and when an RFID device is passed by the reader, the serial number is sent to the BeagleBone through USB, through the ASCII text. All you need to do now is look up the serial number in a database, either on the BeagleBone or a host computer, and if required, grant access to the location.