Excellent and important issue. Ownership in and of itself is not a bar to TN classification - to be a bar the ownership is supposed to rise to the level of controlling - but USCBP (at the "border") and even USCIS sometimes (and more and more) take the position that it is or conclude that if one is a founder or has an ownership interest, one is a "manager" and not an "engineer" and therefore ineligible for TN classification because there's no NAFTA occupation for managers.
Thanks Peter! So then would you suggest to wait on (fingers crossed) getting an H1-B from an employer and then transferring to the new startup?
Not necessarily because the law is clear that a minority non-controlling interest is not a bar to TN classification (and where the law is on your side, it's usually worth pursuing/arguing).