Learn Italian: Chi parla con voi? - Who is speaking with you?

Because it comes after the preposition con (with). Prepositions like con, di, and per are always followed by "tonic" (or stressed) pronouns. Even though voi looks exactly the same as the subject word for "you all", we must use it here instead of the object pronoun vi.

Does voi mean one person or a group here?

voi is always the plural "you" (like "you all" or "you guys"). If the speaker were asking "Who is speaking with you?" to just one person, they would say Chi parla con te? (informal) or Chi parla con Lei? (formal).

Why is the verb parla (he/she speaks) when the sentence contains voi (you)?

Because the subject doing the action is Chi (Who), not voi. Just like in English, the question word Chi is treated as a third-person singular subject (we say "Who is speaking?", not "Who are speaking?"). So, we use parla, the "he/she/it" form of the verb parlare.

The English translation says "is speaking". Where is the word for "is" in Italian?

In Italian, the simple present tense (parla) covers both "speaks" and "is speaking". You don't need an extra word for "is" to describe an action happening right now.