Learn German: Spielt ihr hier zusammen? - Are you all playing here together?

ihr is the informal plural word for "you" (like "you all" or "you guys"). You use it when you are talking directly to two or more people you would individually address as du (friends, family, children).

Why does the verb spielen change to spielt?

Because the subject is ihr. For regular verbs in the present tense, when the subject is ihr (you all), you take the verb stem (spiel-) and add a -t to the end.

Where is the word for "are" in this sentence?

German doesn't have a separate "-ing" tense like English does. A single verb form covers both actions. So, Spielt ihr? translates perfectly to both "Do you play?" and "Are you playing?". You don't need an extra verb for "are".

Why does the sentence start with the verb spielt?

To ask a "yes/no" question in German, you just swap the subject and the verb. The statement Ihr spielt (You all play) becomes the question Spielt ihr? (Do you all play? / Are you all playing?).