I guess it is a question of your theory of the political process: Is it an perspective with a focus on institutions and possibly veto player (Tsebelis)? Learning? Discourses? Agenda setting? Diffusion? Networks? Rational actors? I very much appreciated an edition by Paul Sabatier: Theories of the political process - which gives an excellent overview on different theories. I guess it would be possible to develop some ideas on the role and importance of different actors - this certainly depends on your view on the process.
It seems like you've answered your own question, so I'm not sure what exactly you're looking for Ahmet?
What context are you talking about when wanting to learn more about who's affecting public policy?
On a general level--everyone in the country (or world) is effecting policy at the national (or international) level, at least in democratic countries. After all, the people have a say, and the politicians are supposed to adhere and design policies that their constituents want.
I agree with Michael's points. Actually, who is affecting policy-making depends very much on how the policy (or policy agenda) is framed and the political context. In countries with a high degree of censorship like China, media influence is not as great as their democratic counterparts.