There is a sign hanging outside the DADA classroom, under the rules and near the homework board.

"Attention Students! If you are interested in joining a Defense organization established by Professor Rae, please read the following:

Professor Rae will be meeting during the course of the term with students who would like to begin their Defense training. Subject matter will include a variety of spells and skill sets that are included in actual Auror training.

If you are interested, please PM the professor and she will add your name to the member list. Points will be given to those members who manage to practice and master these spells and skills, and at the end of the term, those who earn enough points will be rewarded with a medal and the title of Auror-in-Training."