Scrum has only three roles: the Product Owner, the Scrum Master, and the Team. The Scrum Master serves as a facilitator for both the Product Owner and the team. The Scrum Master has no authority within the team (thus couldn't also be the Product Owner!) and may never commit to work on behalf of the team.
There are three roles in the Scrum method of agile software development: the Product Owner, the ScrumMaster, and the team. In Scrum, the Product Owner is the one person ultimately responsible for the return on investment (ROI) of the product development effort. The Product Owner influences the development effort by conveying his vision to the team(s) and prioritizing the Product Backlog.
While backlog refinement (also called grooming) was not originally a formal meeting in the Scrum framework, Ken Schwaber, who founded Scrum, advises teams to dedicate five percent of every sprint to this activity. As with Scrum’s other meetings, the grooming should take place at the same time and place and for the same duration each sprint. Everyone attends the backlog refinement meeting: the team, the Product Owner, and the ScrumMaster.