Who is responsible for creating money within a country, and how frequently does this occur?

Its mostly the Central Banks. Government may or may not have a say in this (depending on which country in question we are talking about). Companies don't have any say in the creation of money directly as a policy. Maybe via lobbying (and that is a stretch too).

Universally, governments borrow money from the Central Banks. If the borrowing demand increases, then to ensure adequate money supply, the Central Bank will have to create more money and inject it into the existing money supply, thereby reducing the buying power of the prevailing currency.