"Who is together with you?"
Translation:Siapa yang bersama kamu?
"Siapa bersama kamu?" is wrong because Indonesian uses "yang" to mark the compliment after any question word that needs further information (siapa and apa). This doesn't make sense in English, so just think that if a verb is needed after "siapa" and "apa", use "yang" to connect them, thus "Siapa yang bersama kamu?". It can literally be translated to "Who is it that is with you?"
I was wondering about yang being there, as tips about yang say only "yang expands a previously mentioned idea". I figured it can be roughly translated as "who is [that one that ~~ yang] is with you?" - and it helped me to remember that yang...
Although it still doesn't answer why "siapa bersama kamu?" is wrong.