"Nein, nicht mit euch."
There are several words for the English "you" in German. Therefore, "with you" can be translated in three different ways:
- mit dir (singular, informal)
- mit euch (plural, informal)
- mit Ihnen (singular & plural, formal)
You have to use the dative case after the preposition "mit". That's why you can't use "dich" (accusative) but have to use "dir" (dative) or one of the other two options (euch, Ihnen), which are also in the dative case.