I would like to believe that it is not because of late coming that they sit there. To answer your question, first you must educate them - That sitting at the choir stand is like occupying an office. The only difference is that this is of a high calling. It doesnt matter which Christ Embassy I go to, I sit with the choir. It is my rightful place. And in all honesty when they sit in the congregation they must be uncomfortable. That is not where God put them. Then after teaching them observe if they will still sit there. If they do, then it means they are not serious about being in the choir.