The whole network's distributed and nobody is in charge of making decisions. The majority of people running network nodes decide whether to accept a change or not by upgrading the Bitcoin software they use to a new version containing the change.
There is a group of developers who code changes into the software, but anyone can suggest changes they could make, and if they approve the developers will include the change in the next software version they produce.
There is a group of developers who code changes into the software, but anyone can suggest changes they could make, and if they approve the developers will include the change in the next software version they produce.
Keep in mind that there is no "The Software". Anybody can make any change to their own code. The trick is to get others to make the same changes. Right now, most bitcoin developers tend to follow the code in Bitcoin Core, but that could change (perhaps even in a few months if enough people switch to Bitcoin XT).
