Owner of passthrough corporations must establish an actual outlay of resources to establish sufficient basis in the corporation to list the corporations losses on their taxes. Signing as a co-borrower a corporate loan does not establish a personal outlay.
[again, folks, this site doesn't contain advice of any kind.]
http://media.ca8.uscourts.gov/opndir/18/06/171694P.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.