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.