I read this book some time ago.
The story is about two men and two women who are abducted by aliens. They meet each other on a space ship. Later on, they are released onto a planet where they have to survive. They meet also four other aliens (two male and two female). When it comes to a fight, the people from Earth win but they don't kill the aliens.
Then the other aliens come and tell them that Earth will not will be destroyed, and that they can stay on the planet or will be brought back to Earth.
They stay, and the alien say the other test subjects from Earth in other places also stayed.