In Russian, I know there is a word similar to "Заткнись" but it is used for a group of people instead of just one person. What is it?
If you wish to use the same word in plural, you actually have two variants.Both are rude.
Заткнитесь! (imperative plural ) and А ну, заткнулись!(past plural used for imperative )
But there's still a great variety of synonyms, like замолкли/замолкните,заглохли, etc. My favourite one is Кончай базар!(used in a singular imperative, but addressed to a group)