Consulta SQL que involucra agrupar por y uniones

Prueba esto (modificado para proyectos sin ofertas):

SELECT
  Project.addDate,
  Project.idOwner,
  Account.Username,
  Project.idProject,
  Project.Price,
  ISNULL(q.offercount, 0) AS offercount
FROM
  (
    SELECT
      o.idProject,
      COUNT(o.idProject) as offercount
    FROM Offer o
    GROUP BY o.idProject
  ) AS q
  RIGHT JOIN Project ON Project.idProject = q.idProject
  INNER JOIN Account ON Account.idAccount = Project.idOwner
ORDER BY addDate DESC