Learn German: Lest die Seite zusammen! - Read the page together!

This is a command directed at multiple people you know informally, which corresponds to the ihr (you all) form. To make an informal plural command in German, you take the present tense verb form lest and simply remove the pronoun ihr.

I thought the verb lesen changes its vowel to ie. Why isn't it Liest?

The vowel change to ie only happens when talking to or about a single person, like du liest (you read) or the singular command Lies!. When talking to multiple people using the ihr form, there is no vowel change, so the stem stays the same as the infinitive, making it lest.

Is die Seite in the accusative case here?

Exactly! The page is the direct object receiving the action of being read, which puts it in the accusative case. Because Seite is a feminine noun, the word for "the" is die in both the subject (nominative) and object (accusative) cases, so it looks like it hasn't changed.

Does zusammen mean they are reading out loud at the same time?

Not necessarily! While zusammen means "together," it can mean reading aloud in unison, taking turns, or just silently reading the same page at the same time. The sentence just tells the group to do the activity together.