No, there isn't.
You can use go to and put your bots in aggressive mode to defend a location, but you can't tell them to follow and bodyguard a moving npc or other player.
Edit: OK, so maybe you don't have robotics primary, but I do, and that's obviously what my brain defaults to 
Warning:
The above post may contain Cynicism, sarcasm and/or pessimism. If you object to the quantities contained, then tough.
Well, you could put them on follow, and put yourself on follow with the character you want guarded.
Thanks for responding, I thought that was the case but thought I'd just check in case I was missing something really obvious.
Quick Question: Does anyone know if there is a defend target command?