Learn Romanian: Ești acolo?

In Romanian, you can usually drop the subject pronoun like tu (you) because the verb form already tells us who is doing the action. Ești specifically means "you are" for one person you know informally, so the "you" is built right into the verb!

Does the word order change because this is a question?

Not at all! Unlike English, which swaps words around ("You are" becomes "Are you?"), Romanian keeps the exact same order as a normal sentence. You just raise the pitch of your voice at the end to show you are asking a question.

If I wanted to ask if a group of people is there, would I still use ești?

No, ești is only for a single person you know informally. If you were asking a group of people "Are you all there?", you would use the plural form of the verb, making it Sunteți acolo?.

How would I change this to ask "Are you here?"

You would just replace acolo (there) with its opposite, aici (here), to make the sentence Ești aici?. Neither of these location words change their spelling based on who you are talking to.