At the end of the game, you'll find yourself in a parking lot surrounded by burning pieces of GLaDOS. A few seconds later, you're dragged away by an unknown speaker that thanks you for assuming the 'party escort submission position'.

This unknown speaker is the 'party associate' or 'party escort bot' as it later became known. It was mentioned by GLaDOS right as you escaped Chamber 19 - she asked you to ''place the device on the ground, then lie on your stomach with your arms at your sides", with the promise that "a party associate will arrive shortly to collect you for your party."

GLaDOS exploding seems to have knocked you unconscious, and it's fair to assume your limp body assumed the position - and was dragged back into the facility for the events of Portal 2.