I don't know if this is the case for interviews or not, but in general, you do not have to sign the NDA to visit Google. If you choose not to sign it, you get a different looking name sticker, and Googlers have to be careful not to show you anything that could be confidential. You're also restricted to common areas; you can't see your friend's desk. If you sign the NDA, you are allowed to see everything (within reason), and Googlers can be a little less guarded around you. But if you just want to have lunch at Google or attend a LUG meeting, you can do that without signing anything. In theory.