Pop songs for an audition?

I presume the audition will also be acapella, so you will need to pick songs with strong melodies that can stand alone. That is, if it's something that requires an digital effects like autotune, or has more instrumentals than actually singing, you don't want to pick those songs. Since you will be singing with other people (eventually), you want to make sure you can sing accurately. Don't be "expressive and emotional" to the point you riff yourself in and out of random keys. This isn't American Idol.

Unless they are also looking to see your ability to sing as a soloist, you should keep in mind that you will be expected to be able to blend your voice with the other singers, so you don't necessarily have to be singing as loudly as you possibly can either. Usually that makes you end up screaming instead of singing.

If you know the general singing style of the group, imitate THAT if you imitate anything at all.

Meanwhile, pick a couple of your favorite songs that you might have been singing for fun, and that you think shows off your voice. Don't worry too much if they don't necessarily sound like the original singer or if they aren't in the original keys. You will be doing choral arrangements of pop songs anyway.

You might even think of contrasting styles of your "mashup". A sad, slow verse from one song to finish up with a high energy, up-tempo chorus? That might be fun. You would have to think about how the songs would fit together logically. Maybe if you were doing love songs, one part would be being dumped and heartbroken, and then yippee! You found someone better or you find you're glad the jerk's gone and that you're independent.

Pick something fun for you to sing. That enthusiasm will carry over into your performance.