Who certifies the press? Who grants credentials?

In controlled societies reporters don't have that luxury. The government licenses them, and that's it. And even in free societies there are instances when badges have to be given out for crowd control or security reasons: covering a country's king, prime minister or president; covering major sports or political events, etc.

Credentials generally are not needed to make appointments or do interviews. Someone who refuses an interview, because you don't have credentials, would probably turn you down anyway.