"Other than you, who else are there?"
Translation:Selain kamu, ada siapa lagi di sana?
I'm not sure about the general rules, but it's said like that.
You can even omit 'ada' , because it's obvious that you're talking about presence/location.
Selain kamu, ada siapa lagi di sana?
Selain kamu, siapa lagi di sana?
Selain kamu, siapa lagi (yang ada) di sana?
They all have the same meaning.