"Băieții au râs de noi stând în picioare."

Translation: The boys laughed at us standing up.

[twovectors]

This is an ambiguous and date I say, meaningless, sentence in either language

[jh963]

What does this mean? Who's standing up?

[razvan.marin]

In Romanian it is ambiguous, though I tend to believe it is us as to make it more like the boys were standing, we would've said: Băieții au râs stând în picioare de noi.

[crbratu]

As Razvan said, it is ambiguous. I read it as "they" were standing. For the other possible meaning I would say "băieții au râs de noi fiindcă stăteam în picioare"

[Coriander18]

So what did the author mean? They were standing up laughing at us or they were laughing at us because we stood up?

[Matilda209409]

it is most likely: they were laughing at us while we were standing up. The gerund is just another noun in apposition to noi, there is no causal link.