As a main dish, calculate 100 g (roughly 3.5 oz) and as a side, 60 g (~2 oz) per serving of pasta. At a company picnic people might only take a bit of everything to try, and not a regular "meal size" serving, and not everyone likes everything anyway, etc.
I would actually calculate more like 40 g (~1.5 oz) per person. Also depends on the other ingredients (and how rich/filling they are) of the dish your wife is going to make. 5 lbs sounds reasonable to me too if you want to make sure everyone gets at least one serving.