If you have capable seamstresses and designers on hand, you are blessed. When your show requires contemporary clothing, the cast can provide its own. If you have to rent or borrow costumes, have someone coordinate measurements, costume selection, alterations, and costume maintenance.
Someone has to imagine this, create it and make it comfortable for the actors on the stage - the costumer! Very important people in amateur theatre because they generally have almost no money to do this, but they DO!