You can interface with other BeagleBone or Raspberry Pi exactly as you would connect a BeagleBone to a BeagleBone or a Raspberry Pi:
- network
- MIDI
- I2C
- SPI
- serial
- CAN
- possibly more?
I know nothing about most of these, but any resources you will find online about BeagleBone, or - for any Linux computer - should work out of the box also on Bela.
For the records, the Bela board is a cape that goes on top of the BeagleBone Black (or Green), so when you run something on it you are actually running it on the BeagleBone. Your board has a Debian Wheezy image with kernel 3.8.13 with Xenomai 2.6.