I read this book a few years ago and I want to reread it.
A group of friends plays a video game together. They do a massive feat in the game and then are told that the game is based off reality and they have been selected to fight in this war. One of the friends is accepted while everyone else is turned away.
Not wanting to be split up they enter the realm and go rescue the friend who was accepted officially.
The book was an MMORPG style in English. It was aimed towards older teen-adult audiences. It is a series.