From Wikipedia:
"A shill, also called a plant or a stooge, is a person who publicly helps or gives credibility to a person or organization without disclosing that they have a close relationship with the person or organization. Shills can carry out their operations in the areas of media, journalism, marketing, confidence games or other business areas. A shill may also act to discredit opponents or critics of the organization in which they have a vested interest through character assassination or other means...
Plant and stooge more commonly refer to a person who is secretly in league with another person or outside organization while pretending to be neutral....
Shilling is illegal in many circumstances and in many jurisdictions because of the potential for fraud and damage; however, if a shill does not place uninformed parties at a risk of loss, but merely generates "buzz," the shill's actions may be legal...."