你应该说你经常 和谁 一起玩.
whom就是定语从句的引导词啊,with是它的介词.
简单来说从句分化出来就是,you often play it with whom可以说成You should say who you often played it with 吗with可以后置，后置的时候whom的问题不那么严格，可以用who