What if the world's population suddenly reached 10 billion?

Keep in mind this change will occur overnight. Authorities will have no previous warning nor time to prepare for this great calamity. 

Let's say they're your run-off-the-mill, unremarkably average people. In the case of China, they're regular Han Chinese citizens living perhaps in suburban environments, with basic education, perhaps with an engineering or public job. They're filling, nobody too remarkable. Of course they have clothing, otherwise it would make a very awkward situation. Think of it as if every nation worldwide took a few decades into the future population-wise, without being to concerned for demographic booms and whatnot. Obviously the planet is unprepared for such large-scale jump, otherwise this thread would be useless. Thoughts?

Agreed. Okay, let's say that during the initial chaos a few million people die (that's horrible, but bear with me)
The initial carnage lasts some few months, but authorities somehow are able to keep the situation from a total chaos.Temporary housing is provided, and rationing begins. Every indoors space available becomes housing, especially in urban China and US. During the initial disaster, businesses, residences and buildings are looted, there is anarchy on the streets and services are non-functional. This goes on for several weeks, causing many deaths worldwide. However, the international community is able to prevent a major breakdown of society and provide immediate relief. The UN has an emergency session meeting once the initial disaster stops. World governments agree to provide enough for the newly-arrived 2.2 billion people, setting up an international relief system and basic rationing for everybody. 

Commodities and unnecessary products are no longer produced, as every factory on Earth focuses on churning out food, water, and basic shelter for everybody. Lavish or ostentatious houses and residences are torn down, to make several blocky apartments throughout every space available, causing severe discontent amongst the high class and rich society. Soldiers and police guard the streets. Governments impose curfews and food tokens, giving a rise to international black markets. Major companies break down as their products are outlawed and the economy enters a near state of total war. Recreational places see their spaces used for housing people and producing resources. The result is a strange yet functional state of cooperation, as the Earth is picked dry of its resources and essential elements. In a few years, the world will go back to normal as every individual is incorporated into society. The world is able to hold 2.2 billion people, however, the immediate jump could be nearly disastrous. Nevertheless, humanity endured this time of crisis and come back swinging. 

We'll have to check other options then. This planet will become extremely crowded in a few decades, and this added jump of population may just be the last drop. Luckily, such a jump will pressure world governments to reach a solution, like making space colonies or underwater cities, things like that. Major cities like Tokyo, New York and Mexico City will reach over 120 million citizens, spanning several thousand square kilometers. Less land will be available for farming and producing resources. Places like the Amazon and the Congolese rainforest will be completely gone, meaning that rainforests may see extinction by this time. New technologies for making the desert and arid locations fertile, most importantly the Sahara and Australian deserts, in order to accommodate the growing population. We can expect an actual, real-life Moon colony or even various space stations by 2050, then. 

Yeah... that's the bad bit. Contamination will reach a disastrous level of severity. Every major water body, including lakes, rivers, oceans and seas, will be completely full of trash and unable to harbor life. Forests, jungles and natural spaces, completely ravaged by contamination, dry and dead. 99% of all wild species will go extinct, leaving only the useful, domesticated species such as cows, pigs and chickens. The enormous amount of trash and waste covers an area the size of Europe, littered around the seas, mountains and valleys. Air contamination is such, humans would need to live using respiration devices, or within domed cities, until the technology allows to genetically-modify humans to breathe contaminated air. It will be a shame, and the planet will become unlivable soon enough. 

They're random people, nobody too exceptional. Depends where they appear. For example, someone new from India may be a regular Tamil farmer folk living in a rural village in Western India, without much education nor skills apart from farming. It depends on the racial diversity of the country they're on. For example, 91.5% of China's population are ethnic Han Chinese, so 91.5% of the people that appear in China will be Han Chinese. Since they're slightly more men than woman worldwide, perhaps 48.9% of the 2.7 billion will be woman, and the rest will be males. Education also depends on the country. Few people are professionally educated in Zimbabwe, for example, so few new people in Zimbabwe will have professional education. It is proportional to the country they appear on on every sense.