This seems like an awkward phrase. Can anyone give a broader context in which this exact phrase would occur?
"Nobody could complete the team challenge until us."
that doesn't make much sense either. "Nobody could complete the team challenge except us" would sound a lot better.
i'm guessing this isn't the english phrase "something is UP TO someone", is it? can we get a native over here?
Okay, so it's "until us" in the sense of "until we did it".
But it has no meaning without a context.