You should prepare about one verse and a chorus of a song that
a. you are comfortable with
b. you think some of us in the group will be familiar with.
Other than that, we'll test your vocal range (have you sing up and down so we can see how high and low you can sing), and we'll test your tonal memory (how accurately you can remember sequences of tones).
If you get a call-back, we'll have you learn sections of a few songs and sing with the group so we can check how well you blend and how quickly you can learn a piece.
If you have any other questions, feel free to shoot any of the members an email or reply on the forums!
Thanks for your interest,