A Secret Service agent was presumably on the plane. As a former First Lady Hillary gets lifetime protection.
An acquaintance who is a bodyguard for a high government official says that a bizjet is probably cheaper for the taxpayer than an airline ticket even if the government picks up the tab for the plane since fewer agents are involved and there is less public exposure.
Hillary is reportedly laying the groundwork for another presidential candidacy and she has been taken to task earlier by activist supporters for the carbon credits she burns on a bizjet.