1) Look at companion
2) Press "E" (or w/e the console equivalent is - it'll prompt you)
3) Look away from the companion
4) Press "E" again
5) Watch as companion moves
That said, I've found the AI to be relatively smart. If they're blocking a doorway and I walk towards them, they're instinctively stepping back for me. And to be fair, the Command feature with companions isn't covered well (or possibly at all). On one hand, it's nice that BGS doens't hold your hand in this game, but on the other hand, there are a ton of hidden features/mechanics that most people don't know about.