Looking at the Schematic, it appears that the board is using a Atmel 328 chip just like the Arduino Uno. This chip is running at 5volts and appears to have the RX an TX broken out to some pins, the USB FTDI, and the Xbee socket. You can connect the Teensy to these pins, but you will need a Logic Level Converter similar to this one: https://www.sparkfun.com/products/12009