Learn German: Lest bitte laut! - Please read loudly!

This is a command directed at multiple people you know informally. To make this, take the normal ihr (you all) form of the verb lesen, which is ihr lest. Then, simply drop the pronoun ihr. That leaves you with the command form: Lest!

Why isn't there a pronoun like ihr or Sie in this sentence?

For informal commands to a group (the ihr form), the pronoun is completely dropped. If this were a formal command to people you address with Sie, you would keep the pronoun and say: Lesen Sie bitte laut!

Why is bitte in the middle of the sentence instead of at the beginning?

While you can perfectly well say Bitte lest laut!, it is extremely common and natural in German to place bitte right after the verb when giving a command.

Does laut mean loud or loudly here?

It means loudly here, because it describes the action of reading. In English, we usually add -ly to make an adverb, but in German, the adjective and adverb share the exact same form. So laut can mean either loud or loudly depending on the sentence!