You have to add them as peers (right click can do that - usually something like IP:PORT). I don't know if there is an API or scripting interface that allows it.
You might be able to do this through the web admin, once the computer have all picked it up and started. Even if they have been added, it won't likely maintain a constant connection regardless, just send/receive packets as needed. A more interesting question is how to do you get n computers to divide the download in roughly n parts (assuming all have plenty of upstream bandwidth available) and prioritize pieces to optimize your download (hint: you can't without writing your own torrent client to my knowledge).
But more importantly, your computers should just automatically discover each other through the tracker or DHT. This is probably a pointless exercise.