The fact that she is a private citizen is immaterial. An investigatory committee of Congress possess the authority to issue subpoenas. This authority has been confirmed by the Supreme Court. See Anderson v. Dunn, 19 U.S. 204; Jurney v. MacCracken, 294 U.S. 125; McGrain v. Daugherty, 273 U.S. 135.
It’s all a matter of timing:
Move too soon and Obama will just give her/them a presidential pardon. Delay until charges are filed toward his exit without a conviction during his presidency and Hill-Billy both wear orange. I’ll be patient! It’s worth every second!