Learn Dutch: Jullie kantoor is heel mooi. - Your office is very beautiful.

Yes, it means "you" as well! In Dutch, jullie is used for both the subject pronoun ("you" plural) and the possessive adjective ("your" plural). Unlike the singular forms (jij versus jouw), the plural form stays exactly the same.

How can I tell if jullie means "you" or "your" in a sentence?

You can tell by the word order. Because jullie is placed directly in front of the noun kantoor (office), it acts as the possessive "your". If it meant "you", it would normally be followed by a verb, like in Jullie hebben een kantoor (You have an office).

Does the adjective mooi need an -e at the end here?

Not in this sentence! When an adjective comes after a verb like is, it never gets an extra -e. You only have to worry about adding an -e when the adjective comes directly before a noun, like in het mooie kantoor (the beautiful office).

Could I use erg instead of heel to say "very beautiful"?

Absolutely. Both heel and erg mean "very" and they are completely interchangeable here. Saying Jullie kantoor is erg mooi would mean the exact same thing.