"Είσαι έτοιμη;"

Translation: Are you ready?

[np8gWHZu]

Είστε έτοιμες (feminine plural) / είστε έτοιμοι (masculine plural).

[Dmatafonov]

how do I ask the same in you (plural)?

[DashAlex22]

Aye aye, captain!

[Araucoforever]

Am I correct to assume that the person to which this question is addressed is a woman?

[D_..]

A woman or a girl, yes.
I think you'll find looking things like this up in Wiktionary very useful, it usually includes grammatical gender information. :)

[IWL]

Έτοιμη refers to someone in the feminine. Since a girl, κορίτσι, is neuter, why wouldn't it then be έτοιμο?