About 5.2 million Americans are currently underemployed, defined as those who are seeking full-time work but receive fewer than 30 hours per week from their employer. In addition to this segment, there are those who work full-time jobs but are unable to find companies that offer flexible hours to pick up additional shifts. Wonolo, an online staffing marketplace, is trying to fill this gap by connecting companies with interested workers. The San Francisco-based startup announced today that it has raised $13 million in a round led by Sequoia Capital, with partner Jess Lee joining as a board member.