There's a concrete reason you see very few Googlers working part-time with startups. Google has a conflict of interest policy that disallows employees from working on competing products; since Google has entered so many areas of technology recently, almost anybody can be considered a competitor. On the other hand, I've seen a lot of my friends leave Google recently to pursue side interests.

You can decline the NDA and nothing bad will happen. All visitors are asked to sign an NDA, which is a silly policy but easy enough to ignore.