Learn Dutch: Blijven jullie bij de deur? - Are you staying by the door?

The preposition bij means "by", "near", or "at" a location. Because the people are staying in a stationary spot rather than moving toward a destination, you use bij. If they were walking toward the door, you would use naar instead.

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

Dutch doesn't use an "are ...-ing" structure like English does for things happening right now. The single verb blijven covers both "stay" and "are staying". So, Blijven jullie? translates perfectly to "Are you staying?".

Why is the verb Blijven at the very beginning?

To ask a yes/no question in Dutch, you simply swap the subject (jullie) and the verb (blijven). You don't need extra helper words like "do" or "are" at the start. The statement Jullie blijven (You stay) simply becomes the question Blijven jullie? (Do you stay? / Are you staying?).

How would this sentence change if I were only talking to one person?

Jullie is strictly plural, meaning "you all" or "you guys". If you were asking just one person, you would use jij (informal) or u (formal), and the verb form would change too. You would say Blijf jij bij de deur? or Blijft u bij de deur?