You can also be prosecuted for lying, since the polygraph examiners are federal investigators.
That part is not true. Polygraph examiners are not sworn federal law enforcement officers. But they are happy to refer any of your admissions of guilt to federal law enforcement who may come ask you the same questions. Lying to them is a criminal offense.
Now that I say that, though, I can't find a reference to support this.
More precisely, it's a violation of 18 USC 1001 to lie about "any matter within the jurisdiction of the executive, legislative, or judicial branch of the Government of the United States" (with a couple of specific exceptions as given). Technically you don't even have to lie to a Federal official since the statute doesn't specify that; it just has to be a matter "within the jurisdiction" of the Federal government.
In my case, I was extremely open and honest with the main investigator that I was interviewed by, including the things I did during college with recreational pharmaceuticals. I told all my friends to just "tell the truth" about what they saw happen.
I think my biggest benefit there was telling the story about how I saw that kind of stuff tear lives apart, and turn people who had been best friends into worst enemies, and the negative follow-on impacts of what happens when you can't pay your rent, you can't pay your electric bills, etc... because you have this debt to someone else that suddenly must be repaid this afternoon.