The precise translation to your question is "Ποιος από αυτούς είσαι;" Which one of the people you see. But yes, in a simple discussion you could also say that.
I am not a native speaker, but usually in languages like this the masculine version is dominant and the feminine is only used when it is clear that whoever is meant is female (or a female only group). However in this case both should be correct in my opinion.
In koine Gk this would be τίς, as in τίς εἶ, "who are you?" but then would transition to Ποιος when asking about the person's family background, e.g.,ἐκ ποίας πατρίδος σὺ εἶ, "from what/which family are you?"
Οι is pronounced 'ee'. Check the Diphthongs notes in the Tips and notes section under the Alphabet skill Tips and notes are available on the website but not on the apps.