Once again we face translating a verb of NO motion combined with a directional adverb that implies motion, and something has to give in the English. If this sentence doesn't mean "No, I won't go and stand behind them" then I can't think of an idiomatic sentence in English that it does translate as. Reported.
I think "I'm not going to stand behind them" or simpler, "I won't stand behind them" is what you'd say in this situation.
This "őmögéjük" sounds a bit awkward even in Hungarian and we usually use it in very limited situations. While it is completely valid literally, it is usually shortened to "mögéjük" in spoken language. But it is still important to have it in the course because you must recognize that in "Nem, mögéjük nem állok" has this reference to them and it is "őmögéjük" in its full form.