We can either say “να γράψετε” or “γράψτε” (= write!), both of them are correct. Please note that “να γράψτε” is incorrect.
“να γράψετε” is a form of conjunctive in second plural and “γράψτε” is a form of imperative in second plural.
Both phrases mean exactly the same thing (= write!). However “γράψτε” sounds like an order, it is more firm. “να γράψετε” is a call to action and is more polite.
We can use imperative or conjunctive to give an order, the imperative though is usually more firm and emphatic.