How do new Chinese characters get added to the language? For example there are characters such as 熵 which as far as I know only means one thing - in this case entropy - and which have to have been deliberately added by some person or organization. An earlier example is the character 逛, which according to evidence in manuscripts of Dream of the Red Chamber was added to the language fairly recently, in the 18th century.

My question is: who / what at presently decides what new characters get added to the language? If we were to petition some one or thing to get the 招財進寶 rebus added to the language as an actual character, who would we petition? And do new characters added to the language have to have a pronunciation?
国务院, or the State council, decides formally on official characters, for example characters for new chemical elements (like 钐), species (like 鳤) or physical properties, like your example 熵.

But the decisions in China are made just like they are in the rest of the world, that is, they adapt to actual usage. Some characters go, some come.

If a character becomes popular, the government will most likely incorporate it into the 通用规范汉字表 (see http://www.gov.cn/zwgk/2013-08/19/content_2469793.htm for the last edition of 8105 characters).

Names like 喆 (Zhé) and 淼 (Miǎo) are now part of the official standard, although they have of course existed all the time. There is also the classical example of premier 朱镕基, where 镕 was not part of the standard. This is fairly common in China, as odd name characters exist in abundance.

Sometimes existing characters are modified to contemporary usage, like 囧, but is mostly slang and will often not make a permanent mark in dictionaries.