Learn Russian: Итак, вы согласны?

While Russian does have verbs for agreeing (like соглашаться), expressing "I agree" or asking "Do you agree?" in the present tense is almost always done using a short-form adjective. You are literally asking, "Are you in a state of agreement?" Short adjectives are typically used for temporary states or conditions, making them perfect here.

Why does согласны end in -ы?

Short adjectives must agree in number and gender with the subject. The subject here is вы, which is grammatically plural. Therefore, it takes the plural ending -ы, even if you are using вы politely to address just one person.

If I were asking a close friend instead of using вы, how would this sentence change?

You would use the informal ты. Because ты is singular, the short adjective must match your friend's gender. You would ask a male friend, Итак, ты согласен? and a female friend, Итак, ты согласна?

What is the difference between starting this sentence with Итак versus just Так?

Итак is specifically a concluding word. It is used to summarize, draw a conclusion, or get to the final point after a discussion (meaning "So then..." or "Therefore..."). Так is a much more general word for "so" or "this way" and doesn't carry the same "let's wrap this up" feeling that fits a negotiation.