I've seen lots of speculation that there is an NDA. I've heard rumors of a "friend-DA." I've not seen a single person say that they actually had to sign one (and several family member owners have specifically said that they didn't sign an NDA).
There is an NDA. supposed (seeking alpha) NDA for all model 3 buyers/owners | Tesla
It may not be officially called an "NDA" (perhaps 'buyer's agreement' or something like that), but when you can lose your job over breaking the "buyer's agreement", it might as well be called an NDA.
Plus, first rule of NDA Club is "Don't talk about NDA club".