This is an ambiguous and date I say, meaningless, sentence in either language
What does this mean? Who's standing up?
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.
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"
So what did the author mean? They were standing up laughing at us or they were laughing at us because we stood up?
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.