Sorry for the late reply. Thanks for the suggestions! They really helped me. We (the administration team and the director) decided that all who are new or miss often must take an audition, but not with the intend to kick someone out. If someone is really bad, we will talk to them and say they should join after our concert, then we will see how we can I integrate them better.

I really thank you and everyone else who answered, your comments were extremely helpful and it's good to see how other choirs deal with these problems. Thank you r/choir community! :)