Learn German: Öffnen Sie den Koffer!

In German commands (the imperative mood), the verb moves to the first position. For the formal Sie form, you simply use the infinitive (the original dictionary form) of the verb and put it right before Sie, making it Öffnen Sie.

Why is Sie included here? I thought commands drop the "you" word.

That is only true for informal commands (like telling a friend Öffne!). For the formal imperative, which you are using here, you must always include the formal pronoun Sie right after the verb.

Why does it say den Koffer instead of der Koffer?

Koffer (suitcase) is a masculine noun. Because it is the direct object of the sentence—the thing that is being opened—it must take the accusative case. For masculine nouns, the word for "the" changes from der to den in this case.

How would this look if I were telling a group of friends to open the suitcase?

To command a group informally (the ihr form), you use the verb stem with a -t ending and drop the pronoun entirely. The sentence would become: Öffnet den Koffer!