For NSF proposal and grant purposes, do graduate students meet the definition of "investigator"?
Actually, the NSF, in its Grant Proposal Guide, does not actually define the concept of an "investigator." Instead, it chooses to define the different categories as "senior personnel" and "other personnel." Typically, postdoctoral fellows and graduate students fall under "other personnel." 

Personally, in such matters, I would opt on the side of caution and just have the graduate students fill out the conflict of interest form. It doesn't take very long, and having it in place makes life easier for everyone.